Depending on what it’s made of, a coarse file should do it. (mill-
bastard coarse, not #0 swiss coarse. AKA: really coarse.) Failing
that, a belt sander with 80 grit should do nicely.
I mostly don’t worry about the faces of mallets unless they get
totally shredded. What are you doing with them?
Most of the plastics used for mallets wouldn’t respond to torch
supplied heat very well at all. Just grind/file it.
As a suggestion, order a bunch of replacement heads at a go, or
learn to make your own. (depending on the mallet, they can be pretty
trivial to make.) Failing that, solid plastic mallets aren’t that
hard to make either.
Where are you that supplies are an issue?