barry, you will pay as much for a set of 29 wheels as you will 21, none of the other honda 400s will help as they are either 18" rear or single sided and CBR600 will be to wide.
i think 29 wheels will go on but you do have to make a few mods to make em fit.
i think drsnider was selling some 21 wheels a while ago, maybe he still has em
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MC21SP - Roth-tax
CBR400 - track
MC21R - sold
MC21SP - sold