Python - Como implementar un menu con pygame

import pygame, random

WIDTH = 800

HEIGHT = 600

BLACK = (0, 0, 0)

WHITE = ( 255, 255, 255)

GREEN = (0, 255, 0)

RED =(255,0,0)

pygame.init()

pygame.mixer.init()

screen = pygame.display.set_mode((WIDTH, HEIGHT))

pygame.display.set_caption("Pokemon")

icono= pygame.image.load("assets/poke.png")

pygame.display.set_icon(icono)

clock = pygame.time.Clock()

def draw_text(surface, text, size, x, y):

	font = pygame.font.SysFont("serif", size)

	text_surface = font.render(text, True, RED)

	text_rect = text_surface.get_rect()

	text_rect.midtop = (round(x), round(y))

	surface.blit(text_surface, text_rect)

def draw_shield_bar(surface, x,y, percentage):

	BAR_LENGHT = 100

	BAR_HEIGHT = 10

	fill = (percentage / 100) * BAR_LENGHT

	border = pygame.Rect(x, y, BAR_LENGHT, BAR_HEIGHT)

	fill = pygame.Rect(x,y, fill, BAR_HEIGHT)

	pygame.draw.rect(surface, GREEN, fill)

	pygame.draw.rect(surface, WHITE, border, 2)

class Player(pygame.sprite.Sprite):

	def __init__(self):

		super().__init__()

		self.image = pygame.image.load("assets/p3.png").convert()

		self.image.set_colorkey(WHITE)

		self.rect = self.image.get_rect()

		self.rect.centerx = WIDTH // 2

		self.rect.bottom = HEIGHT - 10

		self.speed_x = 0

		self.shield = 100

	def update(self):

		self.speed_x = 0

		keystate = pygame.key.get_pressed()

		if keystate[pygame.K_LEFT]:

			self.speed_x = -5

		if keystate[pygame.K_RIGHT]:

			self.speed_x = 5

		self.rect.x += self.speed_x

		if self.rect.right > WIDTH:

			self.rect.right = WIDTH

		if self.rect.left < 0:

			self.rect.left = 0

	def shoot(self):

		bullet = Bullet(self.rect.centerx, self.rect.top)

		all_sprites.add(bullet)

		bullets.add(bullet)

		laser_sound.play()

class Pokebol(pygame.sprite.Sprite):

	def __init__(self):

		super().__init__()

		self.image = random.choice(pokebol_images)

		self.image.set_colorkey(WHITE)

		self.rect = self.image.get_rect()

		self.rect.x = random.randrange(WIDTH - self.rect.width)

		self.rect.y = random.randrange(-140, -100)

		self.speedy = random.randrange(1, 10)

		self.speedx = random.randrange(-5, 5)

	def update(self):

		self.rect.y += self.speedy

		self.rect.x += self.speedx

		if self.rect.top > HEIGHT + 10 or self.rect.left < -40 or self.rect.right > WIDTH + 40:

			self.rect.x = random.randrange(WIDTH - self.rect.width)

			self.rect.y = random.randrange(-140, - 100)

			self.speedy = random.randrange(1, 10)

class Bullet(pygame.sprite.Sprite):

	def __init__(self, x, y):

		super().__init__()

		self.image = pygame.image.load("assets/rayito1.PNG")

		self.image.set_colorkey(BLACK)

		self.rect = self.image.get_rect()

		self.rect.y = y

		self.rect.centerx = x

		self.speedy = -10

	def update(self):

		self.rect.y += self.speedy

		if self.rect.bottom < 0:

			self.kill()

class Explosion(pygame.sprite.Sprite):

	def __init__(self, center):

		super().__init__()

		self.image = explosion_anim[0]

		self.rect = self.image.get_rect()

		self.rect.center = center

		self.frame = 0

		self.last_update = pygame.time.get_ticks()

		self.frame_rate = 50 # VELOCIDAD DE LA EXPLOSION

	def update(self):

		now = pygame.time.get_ticks()

		if now - self.last_update > self.frame_rate:

			self.last_update = now

			self.frame += 1

			if self.frame == len(explosion_anim):

				self.kill()

			else:

				center = self.rect.center

				self.image = explosion_anim[self.frame]

				self.rect = self.image.get_rect()

				self.rect.center = center

def show_go_screen():

	screen.blit(menu, [0,0])

	draw_text(screen, " Pikachu debe sobrevivir ",40, 400, 30)

	draw_text(screen, "Press Key", 30, 435, 450)

	pygame.display.flip()

	waiting = True

	while waiting:

		clock.tick(60)

		pygame.init()

		for event in pygame.event.get():

			if event.type == pygame.QUIT:

				pygame.quit()

			if event.type == pygame.KEYUP:

				waiting = False

def pausa():

    pausado=True

    pygame.mixer.music.pause()

    while pausado:

        for event in pygame.event.get():

            if event.type==pygame.KEYDOWN:

                if event.key==pygame.K_c:

                    pygame.mixer.music.unpause()

                    pausado=False

                #if event.key==pygame.K_r:

                    #show_go_screen()

        screen.fill(BLACK)

        draw_text(screen,"PAUSED",100,400,30)

        draw_text(screen,"PRESS c TO CONTINUE",50,500,300)

        pygame.display.flip()

        clock.tick(60)

pokebol_images = []

pokebol_list = ["assets/great3.png", "assets/master2.png","assets/safari1.png","assets/poke1.png","assets/ultra2.png"]

for img in pokebol_list:

	pokebol_images.append(pygame.image.load(img).convert())

####----------------EXPLOSTION IMAGENES --------------

explosion_anim = []

for i in range(6):

	file = "assets/regularExplosion0{}.png".format(i)

	img = pygame.image.load(file).convert()

	img.set_colorkey(BLACK)

	img_scale = pygame.transform.scale(img, (70,70))

	explosion_anim.append(img_scale)

# Cargar imagen de fondo

background = pygame.image.load("assets/back.jpg").convert()

menu = pygame.image.load("assets/menu2.jpg").convert()

# Cargar sonidos

laser_sound = pygame.mixer.Sound("assets/pika.ogg")

explosion_sound = pygame.mixer.Sound("assets/explosion.wav")

pygame.mixer.music.load("assets/music1.ogg")

pygame.mixer.music.set_volume(0.9)

#pygame.mixer.music.play(loops=-1)

#### ----------GAME OVER

game_over = True

running = True

while running:

	if game_over:

		show_go_screen()

		game_over = False

		all_sprites = pygame.sprite.Group()

		pokebol_list = pygame.sprite.Group()

		bullets = pygame.sprite.Group()

		player = Player()

		all_sprites.add(player)

		for i in range(8):

			pokebol = Pokebol()

			all_sprites.add(pokebol)

			pokebol_list.add(pokebol)

		score = 0

		pygame.mixer.music.play(loops=-1)

	clock.tick(60)

	for event in pygame.event.get():

		if event.type == pygame.QUIT:

			running = False

		elif event.type == pygame.KEYDOWN:

			if event.key == pygame.K_SPACE:

				player.shoot()

			if event.key == pygame.K_p:

				pausa()

	all_sprites.update()

	#colisiones - pokebolas - rayos

	hits = pygame.sprite.groupcollide(pokebol_list, bullets, True, True)

	for hit in hits:

		score += 10

		#explosion_sound.play()

		explosion = Explosion(hit.rect.center)

		all_sprites.add(explosion)

		pokebol = Pokebol()

		all_sprites.add(pokebol)

		pokebol_list.add(pokebol)

	# Checar colisiones - jugador - pokebolas

	hits = pygame.sprite.spritecollide(player, pokebol_list, True)

	for hit in hits:

		player.shield -= 25

		pokebol = Pokebol()

		all_sprites.add(pokebol)

		pokebol_list.add(pokebol)

		if player.shield <= 0:

			game_over = True

			pygame.mixer.music.stop()

	screen.blit(background, [0, 0])

	all_sprites.draw(screen)

	#Marcador

	draw_text(screen, str(score), 25, 770, 10)

	# Escudo.

	draw_shield_bar(screen, 5, 5, player.shield)

	pygame.display.flip()

pygame.quit()

# soy nueva con Pygame , ya miré videos  y lei de todo, no se me ocurre que más hacer, si alguien me da consejos como implementarle un menu con botones se los agradesco. Mi MENU actualmente seria la funcion SHOW_GO_SCREEN()