If the engine runs good, probably the biggest thing that could be wrong with it is that the whole rear end might need to be rebuilt. Just look at it like the whole rear end is bad and see what it would cost to get it fixed. If it's more than you want to spend, then it might be a good ideal to move on.
Most 4-wheeler problems are caused by a loose nut connecting the handlebars and the seat!!
You only need two tools in life -- WD-40 and duct tape. If it doesn't move and should, use the WD-40. If it shouldn't move and does, use the duct tape.