Razor. The guys have given you great ideas with the A14/A15 engine conversion.
I also wanted to do it in my 1200GX but due to cost I am going to stay at my A12GX and do some upgrades to it.

VERY IMPORTANT: Work out how much everything is going to cost before you start and then decide if you still want to do it.
That is what I did and saw it is still going to be too much for me.
If you stay with the A12 may I suggest the following:
32/36 weber or twin 40mmDCOE webers.
Gasflow and ported head.
Performance cam. 276 degrees to be still drivable in town.
That will up performance much more over standard form.
Good luck