Any of those motors should be a good match for your US40. We've used the OS 46, OS 50 FSR (no longer manufactured), and OS 50 SX with very good results. For a little more power, the 50 SX has a slight edge over the 46... but for overall reliability and price, it's difficult to beat the 46 FX.

-Juhan

ps: http://archive.ncsa.uiuc.edu/Cyberia...l/rc_us40.html