Can I assume that the main use of this tractor is farming? You probably don't want anything too wild? Want to keep it rebuildable?
You can buy an off the shelf piston rebuild kit with liners sized to fit right in your block that will give you a Super M bore (264 cubes). Most kits will come with dome pistons for a little more compression.
People out there that have switched to a 450 gas or Lp head should be just about giving away the old 8060 Super M gas heads. Add one of these and you will have a high compression Super M motor that is completely rebuildable.
Should give you an honest 50 hp.