Pretty much all the parts for these sleds were interchangablr,the MXZ had a different seat& tank configuration,and different shocks. I think the formula might have had more gauges...(fuel & temp)
A cheaper ,and great improvement for the money,would be to pick up a newer rear skid,you should be able to pick up a sc10-2 or 3 for not alot of money. Upgrade your shocks.You can do some intake work,(rotary valve,carb boots,some jetting,someone must still have this kinda stuff kicking around) a good clutch kit is always MOST bang for the buck. I wouldnt mess around with pipes/cans,you pay way too much for just a bunch of headaches.
A new track,I reccomend a 1" hacksaw (studded or not,its quite an improvment! )The only reason I dont reccomend a 1 1/4 for that sled is you wont ever have any clearance issues with the 1".and if you stud,less issues with the shorter studs.
Trail porting is always an option.