Is there a way to put a 21" rim on a KZ400 hub? or at least fitting up a 21" wheel? Sounds kinda like a goofy question, but I want a bike for tearing around town on and not look dorky. The back tire looks bigger than the front and it looks totally lame.
make sure you have the right amount of spoke hoes! yes hoes! naw really harley's are 40,60,80 spokes depending on application, jap bike are usually some weird even number like 32/36/46, if your lucky maybe a triumph 21" would fit the bill, otherwise call ftm or a rim maker buy the rim from them with the holes drilled for your application. I recently aquired a 18"x5.5" rim blank. If I need a certain number spoke count, Buchanan has a hole cutter, i think it's like $1.00 a hole, 40 min. their lacing is about 80 buck to true and balance the wheel. Their spoke price vary depending on the material and type of spoke.
If you give them a hub and a hand full of bills, they'll make a kawasaki 21", you can also look at those new faux retro cruisers kawasaki makes, you might luck out and they might have a wheel that'll work for you project. but be aware you might have to make a custom axle, or spacers.
IMO I would find a harley or triumph 21" and make it fit with spacers or custom axle.