I'm sorry to be the one to break it to you but you won't be able to build the 420a into some big powerhouse. The best you could do is rebuild it with some strong internals and try to turbocharge it. You will only get a little more power than a stock GST or GSX has though... Not worth the money IMO...
Actually, ya know what you should do. If the car is already semi-gutted and the motor is no good, then you should consider doing a swap. Find a wrecked GST or TSi and get the motor, tranny, wiring harness, ecu, etc out of it and strip this car and put that in instead. It would be well worth the time. Trust me... It won't be easy swapping it all in but you can do it if you get everything and do some research on it.