Another update: there were simply not enough threads left in the original tapped holes for the 12-24 helicoil inserts to stay put. I gave up on that idea, up-sized the two stripped holes to M6 x 1.0 inserts and ordered some M6 socket-head cap screws to use.

I did give some thought to the drilled & tapped aluminum bolt approach, but was concerned the aluminum would be more strip-prone than the carb main body itself. In the end, I decided to go for reliability, rather than being able to maintain the OEM-size bolts.