Hi
Getting a rim to fit depends on hub size and spoke falnge widths as well as spoke number
I had some 21in. wheel rims punched and painted to suit a couple of different front hubs (LE & C10) that I fitted to 20's BSA's as I wanted to have a drum rather than a rim brake
I got "Brickwood wheels" in Salisbury to do these
You might be able to find a 36 x 21 rim from a small capacity trail / trials bike ??
HTH
John