The barrel itself needs very little modfication. The RS/F3 piston is alittle shorter than the NSR one so the barrel needs to have its base and head gasket faces mechined to bring squish, port timing and head volume back into check.

There is also the trouble with what RS piston to use, the later pistons having a longer skirt that will foul a MC21/28 crank. The earlyer type using a shorter skirt but with a off set pin.

The RS piston normaly also uses a 14mm pin, the NSR is 15mm. This means finding a new little end bearing.

Then there is the matter of rings. The RS/F3 ones being just a little over 0.8mm wide if i remember right meaning I'm replacing mine every 150 miles.
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If I have to take the carbs off once more...