Maybe you should start with a top overhaul first. Possibility din sa head ang problem. From there masisilip naman yung engine block kung may problema ang bores, piston rings, or pistons. Reboring and resleeving e normal overhaul process yan. As long as you get the services of a competent mechanic and machine shop. Huwag yung bara-bara kung gumawa. Dapat maingat at malinis magbuo ng makina. Just my two centavos!