Unofficially crankshafts can now [usually] be rebuilt, as the special Koyo bearings have been made available by a few suppliers. However, new cranks are being produced by Honda again, and a rebuild from a reputable builder isn't really much cheaper than buying new. Back in the day a rebuild was a good option, as standard cranks could be quite badly out-of-true, and a good [trued] rebuilt crank could make a noticeable difference in a tuned motor. The new Honda tooling means new cranks being sold today are considerably better than those from 30 years ago, and are built to noticeably tighter tolerances.
Bear in mind that a rebuild will re-use a number of components such as conrods and needle bearings, that may already have [tens of] thousands of KM on them. OEM rollers and cages are available, but are prohibitively expensive, and by the time you add them to the cost of main bearings and the rebuild, the costs far exceed that of a new crank from Honda. Some aftermarket items are available, however they're cross-referenced and not NSR250R specific. We've never used them, and therefore cannot recommend them.
So, I reiterate, do your best to use fuels with the lowest possible ethanol content, as it wrecks '89-'96* centre seals! I would strongly recommend against using the 91, although the NSR will run on it. You won't notice any difference in power between 95 and 97/98 (if you can get it?), however the higher octane rating will be more resistant to knock/detonation, which can occur on the front/lower cylinder on MC21 & MC28 models, and will often be lower ethanol content as a bonus.
* 1987 & 1988 models don't suffer anywhere near as badly, and reproduction '88 centre seals [that are a different size to '87] are available from T2Racing in Japan. The point about re-using components still stands with those cranks though, but there's no "buy new" option from Honda for them, so rebuilding is the only course of action open to owners.
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Andy.
NSR-WORLD.COM
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