Pongゲームに得点システムを追加するには、各プレイヤーの得点を記録する変数を作成し、ボールがパドルを通過したときに得点を更新し、画面に得点を表示する必要がある。ここでは、簡単な得点システムを追加するために、コードをどのように変更できるかの例を示します：

Pythonコード：
```
# Define game variables
ball_pos_x = window_width/2
ball_pos_y = window_height/2
ball_radius = 10
ball_vel_x = 3
ball_vel_y = 3
player1_score = 0
player2_score = 0
font = pygame.font.SysFont(None, 48) # Create a font object for displaying the score

# Game loop
while True:
    # Handle events
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            pygame.quit()
            exit()

    # Update positions
    ball_pos_x += ball_vel_x
    ball_pos_y += ball_vel_y

    # Check for collisions
    if ball_pos_x - ball_radius <= 0:
        ball_vel_x *= -1
        player2_score += 1
    elif ball_pos_x + ball_radius >= window_width:
        ball_vel_x *= -1
        player1_score += 1
    elif ball_pos_y - ball_radius <= 0 or ball_pos_y + ball_radius >= window_height:
        ball_vel_y *= -1

    # Draw the window
    window.fill(black)
    pygame.draw.circle(window, white, (int(ball_pos_x), int(ball_pos_y)), ball_radius)
    pygame.draw.rect(window, white, (paddle_1_pos_x, paddle_1_pos_y, paddle_width, paddle_height))
    pygame.draw.rect(window, white, (paddle_2_pos_x, paddle_2_pos_y, paddle_width, paddle_height))

    # Draw the score on the screen
    score_text = font.render(f"{player1_score} : {player2_score}", True, white)
    window.blit(score_text, (window_width/2 - score_text.get_width()/2, 20))
    pygame.display.update()

    # Delay to get 60 fps
    clock.tick(60)
```

変更したコードでは、player1_score と player2_score という2つの変数を追加し、各選手の得点を記録しています。また、スコアを画面に表示するためのfontオブジェクトfontも作成しました。

ボールがパドルを通り過ぎたとき、ボールが画面の左端を通り過ぎたか右端を通り過ぎたかをチェックしてスコアを更新します。ボールが左端を通過すればプレイヤー2が得点し、ボールが右端を通過すればプレイヤー1が得点します。

最後に、フォント・オブジェクトのrender()メソッドとウィンドウ・サーフェスのblit()メソッドを使って、画面に得点を表示します。
To add a scoring system to the Pong game, we will need to create variables to keep track of the scores for each player, update the scores when the ball goes past the paddle, and display the scores on the screen. Here is an example of how the code can be modified to add a simple scoring system:

Python Code: 

# Define game variables
ball_pos_x = window_width/2
ball_pos_y = window_height/2
ball_radius = 10
ball_vel_x = 3
ball_vel_y = 3
player1_score = 0
player2_score = 0
font = pygame.font.SysFont(None, 48) # Create a font object for displaying the score

# Game loop
while True:
    # Handle events
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            pygame.quit()
            exit()

    # Update positions
    ball_pos_x += ball_vel_x
    ball_pos_y += ball_vel_y

    # Check for collisions
    if ball_pos_x - ball_radius <= 0:
        ball_vel_x *= -1
        player2_score += 1
    elif ball_pos_x + ball_radius >= window_width:
        ball_vel_x *= -1
        player1_score += 1
    elif ball_pos_y - ball_radius <= 0 or ball_pos_y + ball_radius >= window_height:
        ball_vel_y *= -1

    # Draw the window
    window.fill(black)
    pygame.draw.circle(window, white, (int(ball_pos_x), int(ball_pos_y)), ball_radius)
    pygame.draw.rect(window, white, (paddle_1_pos_x, paddle_1_pos_y, paddle_width, paddle_height))
    pygame.draw.rect(window, white, (paddle_2_pos_x, paddle_2_pos_y, paddle_width, paddle_height))

    # Draw the score on the screen
    score_text = font.render(f"{player1_score} : {player2_score}", True, white)
    window.blit(score_text, (window_width/2 - score_text.get_width()/2, 20))
    pygame.display.update()

    # Delay to get 60 fps
    clock.tick(60)

In the modified code, we have added two new variables player1_score and player2_score to keep track of the scores for each player. We have also created a font object font to display the score on the screen.

We update the scores when the ball goes past the paddle by checking if the ball goes past the left or right edge of the screen. If the ball goes past the left edge, then player 2 scores a point, and if the ball goes past the right edge, then player 1 scores a point.

Finally, we draw the score on the screen using the render() method of the font object, and the blit() method of the window surface to display it on the screen.