Holley fuel bowl thread tap

Yes 12-24 sounds right. There's a full list of threads used on Holleys somewhere on RFS. I'll add a link in bit.
There's a few other companies now making quality thread repair inserts (besides Helicoil). I think one is out of Australia. In any event, your closest source of Unified thread stuff is probably the UK.

Deep Roots Holley Thread Sizes and Torque Notes