You can make a swingarm work outta any old stuff. you'll probably wanna consider the type of engine you're using and the axle diameter primarily, and also if it's got a torque arm/lever on the motor, where that might affix. A cool option might be using a monoshock type design from a dirtbike.
Or tbh, cheap available and easy - use a mountainbike rear. the cheap shitty ones are cheap and shitty cuz theyre heavy, but it means theyre usually quite strong, and probably close to what the hub motor was designed to fit. you can then easily go up to a 26" wheel. could ge a whole used Walmart full suspension MTB and weld the whole bottom section on.
Some shit I've done, maybe relevant:
Put a small electric hub motor on a garelli - some details on making the axle fit and other considerations: https://www.mopedarmy.com/forums/read.php?1,4228129
Put a bicycle swingarm on a Trac moped:
http://garage.1977mopeds.com/build/TRACy
https://www.mopedarmy.com/forums/read.php?1,3228109
