By loosing power you mean the travel speed slows, correct? In other words it stays in gear but slows down yet the engine does not appear to be slowing, correct?
Off hand looking at the exploded view I don't see a clutch adjustment. (Edit: Looks like spring #40 is the sole dictator of how much tension is on the drive belt.) But I also had another thought (guess

). Maybe a shear key has sheared either on one of the drive wheel hubs or the transmission drive pulley shaft. One possible way that comes to mind as a preliminary test for this would be to put it in gear with the engine shut off. Then raise
one rear tire off the ground and try to turn it. If you can get it to turn without sliding the other tire while in gear that would be a good sign of a broken key. Being a gear drive, not jumping out of gear, yet slowing down speed wise there has to be some slippage in something external to the tranny. Any internal slippage will make one heck of a grinding sound.