Just about any reg/rec from a 12v bike can be fitted, they all do the same thing; take in whatever power from the stator and chuck out something useable.
The wiring on Honda's tends to be;

Yellow - attaches to the stator feed.
Red - runs into the live feed to/from the battery.
Green - runs to the negative side of the battery.

Plus on some early stuff;

Black - runs directly to ground, i.e. a bolt into the frame.

The MC16 and MC18 both have the reg/rec sat in-line between the charge circuit (from the stator) and the loom, which in reality is how all reg/rec's work.
But on the MC21/MC28 and just about all later Honda's it looks like the yellow wires from the stator run into the loom and the reg/rec plugs in somewhere else. This really isn't the case, the power coming from the stator is death to anything except the reg/rec. Those yellow stator wires always run directly to it.

I spliced an MC18 PGM-II loom to use an MC28 reg/rec, it's just a matter of finding the right plugs and putting it togeather, there can only really be 3 connections, feed from the stator, feed to the positive side of the battery and an earth.
But you have to either butcher your loom to fit a later reg/rec or of you feel clever you could hack your old reg/rec wiring up, get hold of the plug(s) that attach your chosen reg/rec to it's bike normally and make up a little auxillary loom.

Make sure you bolt it to something good, heat is what kills them, especially those rubbish o.e. MC21 and MC28 ones and even more so if you run 55w UK h/l bulbs.
I believe Honda have since superseded them all with a bigger finned thing that doesn't like up on the o.e. bolt holes.
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