the 500 sportsman engine would be fine for the machine but IMO the things you should do are below
1. if you have bushing's instead of bearing's on the axle's, modify to 76 MM bearings, guys on here have good insight on doing it
2 replace the hollow axles with solid splined axles
2. tear down and inspect/rebuild the T20. it can easily handle the HP/TQ of a 500
3. upgrade the sprockets and axle chain's to #50 0-ring chain, Number 60 for the 2 drive chain's from jackshaft to center Axel.
4. start from scratch maybe 2-3k total, but you will have a new machine with years of worry free riding... it will take some time but you will be glad you spent the time now, they are a blast to ride
ALL my .02 cents