You have a flat head engine 8hp check shaft size if 3/4 go with 7 hp overhead valve engine at HF has more torque than flat head engine. Valves in old engine may be stuck I pull head try turn valves if stem want move on one that one be stuck. If get starter cover off maybe able rock engine back fourth. Soak valves that are lifted under face.I got 4 of HF engines since November 2008 one crank rope broke 3 months ago I repair now has short pull but same rope. Save box it comes in if have trouble with new engine box up exchange it. Because regulation they are super lean now but all engine now are that way.