If your going to remove the NSR engine, then it only stands to reason to go with an RS engine. Course you got to go pre right hand drive line (think 91 as Andy mentioned will work).

Don't be screwing a good thing up and putting a 4 stroke in there! (Hey, I still own 3 2 strokes and one is an NSR 28, so I haven't switched teams, just don't have a 2 stroke street bike anymore).
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Charles Gallant