turning an M21 into an M2 or another variant

In a word: NO.  The spare parts left over aren't enough to proceed to a full build of anything else.

While the M4 81mm MMC is based on the M2 hull, you'd have to scratchbuild most of the interior fittings.  Plus, you'd have to source the front cab and the frame and the suspension.  Basically you only have a few M2 walls to begin with.  Given that, your first choice would be to acquire the excellent DML M4 MMC kit (6361)  http://www.perthmilitarymodelling.com/reviews/vehicles/cyberhobby/chc6361.html

The DML M21 MMC is kit 6362, another excellent kit.

If you want to do another DML half track, I think going the cheap route is the wrong path.  Get the full kit.  (I'm on my fifth DML Half track kit, by the way)

Thank you, just brain storming on ideas for my next build. The M2a1 I just built im considering having it captured by the Germans in the Bulge

What would be the best kit to use for making  a british M5 or M9 halftrack?