MC21 and MC28 cylinders (barrels) can be interchanged, as can pistons and rings. I believe the MC28 has (very) slightly milder porting than the MC21, but I need to get my hands on a barrel to make a port-map to confirm this for sure.

Looking at the power delivery graph of a stock MC28 would certainly lead one to assume the ports were just a little more conservative.

Differences between part numbers on the MC21/28 pistons and/or rings is often just Honda updating (superseding) the older part numbers. Simply order MC28 pistons/rings for the MC21.
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Andy.
NSR-WORLD.COM

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