flaking paint/ splitting loco case

You may be past this point in your assessment of the problem, but here goes.

I would be extremely careful about any attempt to separate an original Dorfan engine. All that I have seen would be placed in the "fragile" category due to disintegration of the die cast material. You very easily could wind up with a pile of bits and pieces.

Before I went any further I would suggest having another collector, one preferably with a knowledge of Dorfan products, conduct a "hands on" evaluation of the status of your engine. The issue of paint preservation could also be addressed at that time.

Sometimes, successful preservation means leaving an item alone.

I would agree with Owen.  You say it seems to be "hung up on some of the internals", but that may be all that is preserving the integrity of the locomotive.  Try to take it apart and it really is most likely this will disintegrate in your hands.  The Dorfan castings are notorious, there really is not much you can do about it.  Hard to accept, but that little loco is probably the best it will ever be right now; the more you try to do to it, the worse it will get.  Sad but true.  The Dorfan alloy was a noble experiment, but it failed, and along with it the Dorfan company.  Sic transit gloria mundi.