welllll what I can tell you is the usual cheap upgrade for the NC29 is CBR600F3 shock, when I had my NC29 trackbike apart I compared the standard MC21 shock with my NC29/CBR600 Maxton shock and it looks very much the same but is a good inch longer, bearing in mind that the maxton raises the ride height quite a bit anyway if you fitted this shock to the MC21 it would lean so much from the ridiculous ride height I dont think it would rest on the side stand anymore.
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MC21SP - Roth-tax
CBR400 - track
MC21R - sold
MC21SP - sold