autospeed.com had a nice article on replacing the lilliput ps with a 7812 and a zener. I'll see if I can find it.

UPDATE
I've attached the 7812 based circuit.

First off, I am by no means an electrical expert. Hell, I just know enough to be dangerous. Use at your own risk. You should test the completed circuit with a multimeter before hooking up anything important.

Having said that, I've built this circuit (minus the zener) for other projects and it's always worked fine. Your mileage may vary

Outside of the Zener, everything should be available at Radio Shack.
1) 16v zener diode
1) 7812 regulator
1) 10 uf cap
1) 100 uf cap
1) 5W 1 Ohm resistor
1) Heat sink, bigger is better