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