Wow this is about as easy as it gets why all the fuss
Plane power AL12-70 make sure you reference the Lycoming part number for the old alternator in the log book
Plane power doesn't have STCs they are PMA and the 12-70 comes with all the mounts and brackets or you can get a 12-P70 no brackets and about 100 bucks less.
(O-360-A1A2, -A2A2, -A3A2, -B1A2, -B2A2, -B3A2, -C1A2, -C2A2, -C3A2, -D1A2, -D2A2, -D3A2, -E1A2, -E2A2, -E3A2, IO-360-A1A2, -A2A2, -A3A2, -B1A2, -B2A2, -B3A2, -C1A2, -C2A2, -C3A2, -D1A2, -D2A2, -D3A2, -E1A2, -E2A2, -E3A2)
Log book entry and a revised W&B your off and running

PS maybe you need to shoot your mechanic