to make it longer you will need a replicator from Star Trek to make more links
the drop outs on the R20 are designed for a 3 speed hub
if you replace that with a hub that has a round axle instead of round with flats, you may need to file the drop outs a little.
especially the old Shimano 3 speed hub. Its got a steped axle. So is larger diameter at the hub lock nut. Also 15 tooth is the smallest cog that fits them as the chain hits the hub.
My BSA 20 came with a AG dyno hub on 28 spoke rusty steel 451 rims.
Im going to cut some spokes so I can build it into an alloy 406 20 X 175 rim
there is supposed to be problems with spoke angles with using large flange hubs and small rims. But my RSW has 16" wheels with a front dyno hub.
and nowadays you can get electic motor hubs in small wheels. with spokes that are real short. with a heavy battery to lug about. havent heard of them being harsh to ride. or breaking spokes