You'll probably need to use something to kill the mildew spores inside the case top and bottom as well as the machine.
I've tried quite a few things but for older machines I'm paranoid about the decals and what's left of the shellac clear coat.
Recently I've tried Lysol Disinfecting Wipes on a SEWMORE 128 clone that was covered with mold. Cleaned it right up with no adverse effects. But I have not tried this on an older machine with the Japan type of finish.
I'd start with the case after cleaning your machine. Use the Disinfecting Wipes there first.
Others will have different suggestions, and I'll tune in for those too.
Joe