By SPX I'll assume you mean an Ultra SPX? Heavy, very poor off-trail performance, etc. If you wanna race across a lake or straight sections of trail, your buddies will have to have something real good if they're gonna beat you. But for an off-trail play sled, there are MUCH better options.
1974 Arctic Cat Cheetah 340
Gonna be set up for ice fishing. Should be good for it's purpose!
Most snowmobile problems are caused by a loose nut connecting the handlebars to the seat.