need help getting the max HP out of a bone stock briggs 80202 3hp engine, i bought it for 50 bucks non working, gave it new carb plug and gave it a piston job oil gas and ran. need any help to make it from 4 hp to like 8 hp, under $80 bucks thanks

PS it came off a roto-tiller and it has the sprocket on it, and the shaft is kinda rusted, i got thescrew that holds the sprocked on (it uses a chain #41) and tried to take it off, it was like rusted on, nomatted what tool i used from a pipe wrench to a hammer

thanks! ONY