People have been trying to clean model engines with oven cleaner since they have been made. It has always turned them forever black.

There's nothing that will change this short of dissassembly and glass beading the exterior of the parts.

Even with this operation, the OS engines didn't have a glass bead finish and the engine will never look as original.

You don't know the extent of the damage done in this particular situation. He may have gotten the caustic internal to the engine? if so, it is burned toast.