You should be able to buy direct from Honda in Japan for the MC21/28, as it's part of their Heritage Parts programme.

https://www.honda.co.jp/bike-genuineparts/nsr250r/list.html

Edit: That page no longer shows the crank... maybe because it's fully back in production?

We [Performance Engineering] ordered as many as we could get off the Honda system back in 2015 (I think we got 10), and when we unboxed the first one, it was very clear they were all made with brand new tooling. All the stampings on the con-rods for example, were pin sharp, and the overall finish much more superior. They were also far truer than previous NOS cranks we'd sourced.

I'd still say, get one while you can. I'd also say, if you have a serviceable one, I would most definitely investigate Inoue Boring. See what kind of feedback you can find in Japan for them, because their centre seal replacement looks almost too good to be true! My understanding is a rebuild from them costs about the same as a new crank, but the centre "seal" should last pretty much forever, so you will be able to just replace outer bearings for about £70, if and when they deteriorate... which is very rarely! (Incidentally, this is what we did on the NSR500Vs too, as the centre bearing on those cranks is unobtainable, so replacing the outers frequently put less stress on the centre.)
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Andy.
NSR-WORLD.COM

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