Do you have the old engine removed to get measurements from ? Is your crankshaft 3/4 inch diameter ? How long is the crankshaft ? With your old engine on a flat level surface (work bench ) measure from the level surface to the center of the crankshaft . This information for the Harbor freight engine is in the link I attached above. The bolts for the crankshaft is 5/16 x 24 thread, common 5/16 fine thread bolt if your Tecumseh don't match buy one . If the keyway don't match they sell a step key to over come that. The predator is 3/16 wide

I replaced my Briggs on my pressure washer with this engine with no thinking, I also replaced my Honda on my Log splitter with one of these and The bolts on my Honda where Corse thread and the Predator was fine thread. Take your old bolts with you to try and buy what you need while you are out and about. Get to this point and come back with smiles or frowns and all of us will be glad to talk you through it.