I've rebuilt a few of these engines and they looked very good even with 9K miles on them. We did have one blow a rod through the side of the motor, but I believe that was self inflicted. With no liquid to help them cool, the fan needs to be kept working well and the carbs need to be kept clean or they will overheat and the weakest link will be found. I rode one of ours trying to keep up with the big sleds in Northern Ontario. They would lose me in the straights (which there are a lot of) so my only option was to stay wide open through all the corners to catch back up. Mileage dropped from 11+ to below 8 mpg. This was earlier in the life of one of the motors that I just rebuilt last year at 9K and the pistons were still within tolerance. I replaced them anyway so maybe we will get another 9K out of it.
We have another 2000 PZ-500 that has 8 tooth drivers (11% gear down), a ripsaw track, Viper T-arms, SLP Powder Pro skis, and a Viper skid in it. It really changes that sled off-trail and in loose snow. It really responds now and actually gets better gas mileage than stock ones. I am still trying to believe the mileage part but we have repeated it several times.
Great learner sled, very reliable with a fair to good ride with a little work.
Jetting down wakes them up a bit, but you really need to keep the carbs clean if you do.