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.
Just thinking about this same topic today! Unfortunately, Problem Solvers doesn't make the sliding bottom bracket anymore and it was over $100.00, but someone could probably manufacture one.