Ed is correct in that it will flake off, but if you lightly sand blast it first the paint stays on better than if you just paint over the silver paint or the rust on an old muffler.  

The last time I talked to Mark Korves he said they could not get the black mufflers.  Maybe you can find someone who has a NOS black Nelson muffler.  I would suppose AGCO dealers would be the place to start looking for those.

Bill Brandt