Again, based on the information, it's almost certainly just an MC21 (bike) with an MC28 bottom-end.

There is no difference between the two bottom-ends, other than slightly different gearing in the transmission [if the complete motor was fitted] and the aforementioned GPS/neutral switches.

If you are going to try and convert what you have to an MC28, based purely on the engine number, you are in for an extremely expensive, and not particularly straightforward rebuild!
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Andy.
NSR-WORLD.COM

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