1994 polaris 300 2×4 something slipping in high range?

Hello, I’m looking at buying a 1994 300cc two stroke 2×4 but it’s got a problem I need to know about before I go spending any money. I’ve searched extensively, but the best info I found (although not exactly related to my problem) was here so I thought I’d ask you guys.

The ATV starts and runs fine, low range and reverse are no problem but high range… Well, you can start rolling, and it does go forward, but once you give it much throttle the engine just revs and you don’t get any additional forward motion.

If it were a manual I would say the clutch was slipping, because that’s exactly what it feels like, but as I said, it goes just fine in low range and reverse so I’m rather confused as to what the problem could be (unless it is the clutch and low range/reverse just don’t put enough load on it to cause problems? Or it could be the CVT belt?) If anyone could enlighten me I’d be very grateful. I’ve got until Monday to decide whether to go ahead with the purchase or not.

I’m reasonably competent mechanically so I’m pretty sure I can fix it (if someone can tell me what’s wrong), I just need to factor parts into my purchase price.