I used a value of 192 for the alpha. I also made the rain fall faster now.
I thought that I had removed the shadow successfully in the previous version. Unfortunately I noticed the mistake after I uploaded it. It's fixed now.
I increased the accuracy to the source game now. I made 2 layers for the clouds. The bottom layer moves slower than the upper one, and the clouds move slower overall.
I also fixed mistakes in some parts of the floor, particularly in the right corner.
Edit: now that I think about it, I could separate the floor from the pillars and walls, and make these ones move with a slower delta than the floor. But, in the end, the current version looks more accurate to the source game.
New version available. Download below:
http://www.mediafire.com/download/trwoa483xymy214/ninja_training.zip
Many thanks for your suggestions.