Yes, the bushes are Item 3: Part Number 1179, Nylon Bush.
Because it will drive properly in reverse, but not so well forward, it seems the drive slips in forward only. The design of the automatic tensioner means the tensioner tends to be pulled backward on the mower (which loosens the belt) in forward, but not in reverse. So, there seem to be two possible causes of the symptom you report. The first and more likely possibility, is that the sliding tensioner pulley, located under the seat, needs to be pushed toward the front of the tractor until it clicks into the next ratchet slot. The second possibility, to be considered if the first possibility does not fix the problem, is that the forward clutch is worn out. This something that happens with the Greenfield drive system: the clutch is slipping nearly all the time, and the forward clutch is used much more than the reverse clutch, so it is likely to wear out before the reverse clutch.
