Ext. Alternator Loose?

Without seeing how much material is around the mounting hole that the fastener passes though may be possible that a machining shop could manufacture and press in an insert a sleeve or bush to remove the elongation.

Would be worth checking there is no other damage to the alternator with it being loose such as bearing or warping of the shaft/belt drive.

Suspect a bush pressed in may be a little more cost effective than a whole new alternator.

Is this an Experimental aircraft?  If so, and if the alternator were otherwise airworthy, and if it were my airplane...

I would drill and ream the mounting ear just enough to remove the damage and make it straight and perpendicular to the face of the alternator.  Make the inside diameter of the ear a few thousandths of an inch smaller than the outside diameter of a steel bushing.  Heat the ear to expand the mounting ear and press in the bushing.  Re-mount the alternator with new hardware.  If you don't like pressing steel into aluminum, use an aluminum bushing, TIG-weld the ends, then dress them flat.

Options from McMaster-Carr: Drill Bushings / Spacers / Wheel Reducers

Yes, it's an experimental.  Something like what you guys describe seems like the way to go.  A new alternator seems like overkill.

Sounds like he has no problem spending your money. Explore other options.