I think the gain from a standard a12 to a standard a15 is about 17hp but there is a greater gain in torque.

rear springs you can get reset, i would sujest this before lowering blocks
legally in queensland you can lower your car 30% of its original height, so if you were to reset the springs you would have nothing to show that you have lowered you car

and

in queensland you cant change to an a15, but an a 12 and an a15 visually have next to no difference at all
so you would be sweet with that for the untrained eye of a copper

First the engine; if you run a ported A15 with slightly hotter cam,(something a little hotter than GX cam but more mild than Race or Rally Cam round 260ish duration and .400ish valve lift) , Weber 32/36 and extractors you should see around 100 HP no problem. Keep in mind the stock 1200 gearbox has HUUUUUUGE drops between gears so you want a motor that will pull from 2500 rpm's. Also as noted the 1200 box isn't up to the task of the extra torque of an A15. Find a 120Y box.
As for the suspension; go with 7/8th Front (21mm?)and 5/8th rear (15mm?) sway bars, re-arched rear springs.......for the fronts I think folks in Australia are using the Stanza struts (check Wiki). Some folks will disagree but the 1200 stock brakes are fairly weak,ok for a stocker but once you start getting more power you need better brakes (note I only use my car on the track). Finally get a limited slip diff so the extra power doesn't go to waste. All of the above should put a nice $1000 to $2500 dent in your wallet, based on what you can pick up used.

Tom

if your going the legal way, the limit in qld is 0.2 ltr increase from factory which means a15 is out unless you get a mod plate. but if you dont care about getting one defect go the a15. 1st offence is only a $75 fine just dont let them catch you twice.

Nobody here has ever successfully put an A15 crank into an A12 or 1200 block & made it run.

The A14 crank conversion, however, is not particularly difficult.

Im interested in the rule about not being permitted to drive a car with a mod plate. This means that if a car has suspension, steering & brake system improvements as well as other safety related changes, all of which are approved, your girlfriend can't drive it, even if it still runs a stock engine.

Yep, it must be Queensland.

The original 1200 gearbox is a 56 series while the 120Y gearbox is a 56A series. This means that the internals were strengthened a little but the parent manufacturer still did not use it behind even an A14 engine for good reason.
Have a think about why this is so.

The only way I can think of an a15 fitting an A12 block is by using small crankpin style rods like hayabusa or other bike rods when you offset grind the crank and keep the 82mm stroke or perhaps using hayabusa rods with 37mm ends youll be able to run 84mm stroke. The extra 5mm or more can be overcome with say rods with to suit crankpins of 40mm or lesser. This will allow the crankpins to clear the sidewalls and also I think Chris used an a15 sump?
However the cranks will need machining to stop the pistons hitting or you will have to use slipper style pistons from bikes.