Good link & thanks d gizzle, I'd not seen that before
Thought I'd add my 2 bits - a less expensive & slightly more reliable setup then twin, engine driven freewheel bearings;
https://www.youtube.com/watch?v=U10xLqTekOw
The only freewheel bearing is for pedal power only, so there's little shock or strain on it & the engine can be "bump started" if needful. Most three piece bottom brackets will take external clip 499502H-NR sealed bearings (same as SBP 2 stroke shift kit) if you hone the bottom bracket's threads smooth, they'll press right in. Then it's just a chunk of 5/8" keyed shaft, some lock collars & sprockets of your choosing... the whole thing cost roughly $40-50 tops.