mc21 and mc28 engines are the same as far as fitting and working goes, top ends, bottom ends, heads etc. can be swapped freely.
mc18 I am unsure if a whole engine is a straight swap, mc18 barrels do fit mc21 or 28 cases without issue and vise versa.
standard mc16 bits are best avoided

Early RS (nf5) top ends can be made to work on NSR cases but it's not a straight swap, for starters the power valves are not cabled up in the same fashion, RS pistons are a different pin size, NSR pistons will be the wrong height etc. Potentially some big power there if you get it all working though.
That era RS also didn't run on unleaded fuel.

The later nx5 RS250 stuff (the single sided swingarm RS) is totally different.

The mc28 swingarm is actually lighter than the mc21 one if you put them both on the scales.
The extra 5kg (mc21 to mc28 dry weight difference) is not in the swingarm.

The bad thing about the mc28 one is the rear wheels are damn expensive, the good thing is a wheel change is a one minute job.
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