Crank cases are effectively all the same and other than for originality you're main concern should be making sure the barrels and heads are matched, r barrels/sp heads is a bit high on the compression for normal pump fuel.

se/sp barrels can be distinguished as they are marked with an "L" on the left hand side of the barrel (as it sits on the bike) near the bottom. R barrels have an "H" there

Likewise se/sp heads are marked KV3L on the top, R heads are marked KV3H.

The mc21 flywheel is numbered 037000-2641
The mc28 flywheel is numbered 037000-3321

How do you know the frame is 28sp?

I put a '21 engine in a '28, ran it with a modified '21 airbox and de-restricted pipes but still on the stock card and would guess it made low 50's bhp, the big difference though is the pipes, the '28 ones are restricted with huge washers in the headers, have a look at this picure, the top pipe has had it's restriction removed, the bottom one still has it in place.
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