So I am fairly new to the ATV and UTV servicing. I've never really owned my own Quad until last year and I only got one because it was given to me. So please excuse my lack of knowledge on servicing them.
Anyways, I have a 1997 Prairie 400 (KVF400) and it's had an oil leak since before I owned it. I've investigated the leak and it looks like its coming from the Oil Filter or maybe a nearby gasket? (Hard to tell, then entire area is covered in oil) I've added oil several times and check it regularly to ensure it isn't running bone dry. Last week I let a few of my kids' friends ride it and they drove it for quite a while, while I was BBQ-ing. After about 2 hours I made them come back and park it. After about 20 minutes of it sitting, I got on it and drove it about 10 feet and the acceleration dropped and it made a high pitched squealing noise, similar to a belt squealing. I pulled the cover off the side (where the belt is) and the belt looks practically new. I checked the entire belt over and I didnt see any obvious damage. When the Quad is in neutral I can rev it once or twice then around the third time It squeals and the wheeler dies off like its gonna stall.
I have no idea where to even begin with diagnosing this problem.

I'm hoping someone on here can help me out? Any ideas?