If I ran at horsepower tracks I'd go with the L18 to keep it old school.(vs twin cam) Personally I like the A series engines. Since you can get near 1600cc out of an A series, I would think that an A15 bored out with a upgraded cam and cleaned up head would make between 110HP and 130HP depending on carb/number style. In fact I'm pretty sure I'm finally going to cave and put an A15 in and use my GX head/carbs and upgrade the camshaft.
For me the fun of 1200's is a keeping them what they were. a fun little car that handled well. If you want to turn one into a weapon then don't hold back...............make it a 500hp mid engine thing.