If I were to use some kind of epoxy to rebuild the missing metal, I would
drill a number of shallow holes at an angle using a tiny drill bit to give
the epoxy a good secure bite into the base metal.  In addition, I would mix
in some fine glass fibers with the epoxy to help keep smaller chunks from
cracking  apart and flowing through the cooling system.

That said, I don't think that I'd bother messing with it.  I think that the
only way you'd see any significant loss in cooling capacity is if the
gouging is continuous from the outer portion of the housing to the inner
portion, which would allow pressure backflow into the input portion of the
impeller.

--
Brett in Portland, OR
"Albert" '82 VanaFox I4 Riviera