standard A12 is 68HP GX is 83. Keep in mind the GX engine uses a differant cam it is not a radical grind but better than stock. My guess with just the head would be the 75-78 HP range. You will need the complete GX head with intake and exhaust manifolds.

I would try the Weber 32/36 , header with 2" exhuast , A14 Valves with GX dual valve springs and a hotter cam. You should be able to do that if you shop second hand for carb and header for about $500. That will get you to about 85 HP range maybe 90HP if clean up the head. This is not the trick set-up but good for a street car.

Tom