F3 pistons are single ring (like an RS) and have a 1mm pin offset where the NSR piston is central. Crank price wise, I'm afraid it would be a lot more than what Andy quoted for a rebuilt.

If you already have bearings comming for your old crank I would think it made sence to have it rebuilt and rebuild the engine as it came apart. It will last a good long time if looked after so you wont be in there again for a while.
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If I have to take the carbs off once more...