Hi there Jack, I think I can answer some of your questions & maybe make a suggestion or two. First off, going for maximum horsepower is not a good recipe for a daily driver motor. The old saying is horsepower is revs multiplied by torque. So to get horsepower out of an engine with small torque, like an A12, you have to make it rev. Unfortunately this makes it a pain to drive in traffic, it'll make a lot of noise & will attract the attention of the Police, which is something you want to avoid, especially on P plates. So what I suggest is something like this:-
A12 bottom end with either 76 or 77 mm pistons from an A14
Standard crankshaft & con rods,with little ends broached to take the larger gudgeon pin of the A14 piston
Factory 70 degree camshaft or similar, will rev to 6500-7000 rpm
Twin valve springs
Small combustion chamber A14/15 cylinder head
A14/15 inlet manifold with a 32/36 DGV weber carb
Small 4-2-1 extractors with a 1,75 inch exhaust & muffler
Electronic ignition kit in the standard distributor
New oil pump, water pump & fuel pump
Balanced crank, rods, pistons,etc.
A can of spray paint for the sump,rocker cover & other bits
All this will give you an engine with good torque & good horsepower & will make a 1200 or a 120Y into a bit of a rocket & lots of fun. Just dont take 3 or 4 of your friends along to show them the performance, because the extra weight will slow you down.If you could get away with using an A14 or A15, it'd be even better, but even that might be illegal while your on P's
I've driven 1200's, 120Y's & Sunny's since 1984, & the best street engine was a stock A14 with GX carbys. Lots of torque from no revs, would out drag anything, until the other side of the intersection. The worst one was an A15 with a big cam & a dgv carb. Hard to get moving, thirsty, noisy & a real disaster. Even with twin webers it was a pig. So a nice torquey engine would be my recommendation. One that responds to the throttle, will sit at a set of traffic lights or in heavy slow moving traffic with out overheating, & will last decades with a bit of maintenance. & probably a bit cheaper than the race engine you wanted.
Hope this helps with your project. If you still want to try for big horsepower, it's best to wait til you get off the P's. You can then legally go the turbo route, or the big engine route. The insurance companies will still want huge amounts of money for a policy, though. Just something that never seems to change