Definitely Helicoil it to the original 3/8". You can also reuse the stock bolt that way and keep everything stock. If any of the other holes look suspicious, I'd do them at the same time. Heck, I'd probably just do them all if the engine is a keeper that will see some time with you.
Because you have the holes as a guide, you can do the job under any decent drill press. DEFINITELY not with a hand drill. You may get lucky, but Murphy says otherwise.
Helicoils are a huge asset to threaded holes in aluminum. Puts the holding strength of the thread close to the tensile strength of the bolt.
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