Hpi big block carburetors are notorious for vacuum/fuel leak on the mid-range or jet screw. Causes them to run very hot and changes the tune constantly as things expand and contract. Its probably what toasted your engine. Sometimes they will only leak when the motor is hot so you wont detect it when you're not running. Buy a piston and sleeve set, they come with everything you need and the piston/conrod are already assembled. After you fix that I suggest buying a new jet screw, they are sold with o-rings...don't bother replacing the rings themselves...it wont fix the problem. In some cases the entire carb has to be replaced.