I looked and looked for a new frame that would take a Honda GXH50 and a GruBee transmission. Carried a cardboard cutout and the engine mount into a lot of places. Found frames large enough for the engine, but too large for me. (30" inseam)
Finally Sears / K-Mart had a Schwinn Searcher on sale for $139 and the cardboard would fit as the lower tube bowed out rather than straight or bowed up. Still it is a job to get the mount and engine to fit in the frame and align with the sprocket. Looks like it will work, but takes patience. Best help has been a bike hoist from Harbor Freight to get the frame up to working height.
I am not sure I wouldn't go with a rear 35cc belt drive (Subaru / Robins) if I wan't as large as I am (125 kg, hate #s) so feel I will need the extra power even on our flat bike trails in town.
Good luck, I found that the bike seemed to be fitted up well from K-Mart. The brakes were properly adjusted. Chain in line, wheels adjusted. Looked to me like whoever put it together knew what they were doing.
BTW, Originally wanted disc brakes, but found that the V brakes actually had more stopping power as long as they were dry. Also they would be in the way of the rear sprocket / belt pulley. .Also wanted suspension, but rear ones don't work with a frame / chair drive. OK for the rear belt kits. The Schwinn has a springer front fork and I bought a good quality adjustable spring seat post at a bike shop. ($30). Would suggest you take your time and read, read, read. The guys here and elsewhere saved me a lot of time and money. No sense in reinventing the wheel. Also found believable information on Dax and King sites.
Jim