A steel hardtail makes a good donor bike for road and light trail riding. Those with 68 mm bottom brackets accept front or rear hub systems (read the tutorials at ebikesca) or mid-drives (read about them at Luna Cycle). A front hub conversion is a good place to start since they're easy to convert (with a steel fork and torque arm) and fun to ride. Mid-80's to mid-90's MTB's are good prospects unless you want full suspension. I've never had any problems using bicycles for conversions. They were overbuilt IMO and make very functional systems. Below is one that had a sidecar, designed to accommodate a wheelchair, added. Had a rear hub motor, but is being changed to a dual motor "bike".
