With a top end as old as yours, I assume you would need a bore, thats where they take the cylinders to a bigger size, I suggest you change everthing, pistins rings, bearings.
Just tear down your top end, and have someone measure it and see if its "egging" then figure out what size pistons you'll need to go to, and you can buy a whole kit for about 160 dollars or less. You can talk to Flattrack (a member on the site) or go to
www.magicracing.com or rockymountainatv.com Thats the places I go when I gotta rebuild.