There is no shortage of bso mountain bikes with aluminum rims. They usually range from $5 to free for the whole bike. They have rear hub flanges that are pretty close to the same size as coaster brake hubs so the spokes will transfer over. Yeah, 36h hubs are the factor. So many of the 20" kids bikes have 28h hubs. I'm always looking for those with 36h. Next issue is the kids bikes tend to have pretty short axles so you can either replace the axle and add some spacers to match the frame width or cold set the frame to match. Despite being made in China where metric axles rule in freewheel and cassette hubs, the coasters need a 3/8" axle. I cleaned out my lbs of their old stock years ago. Many of the old 10 speed bikes just work better as single speeders. Simple and reliable bikes that are often 10 pounds lighter.