Howdy WWR, welcome. The HF engine could work but shaft would have to be cut down to fit even in wide cranks. Easy way would be to buy a Honda or HS and a mount kit. Honda is 3 times the cost of HF But I really enjoy mine. Even at top end, it just hums along.
Small Engine Warehouse - Repower Outdoor Power Equipment
and 49cc Mounting Kit Only - $219.99
@BE.com comes with every thing needed to mount including a muffler. Just pointing to an option and not suggesting. Post pics of your build!