I'm actually in the process of doing this.  I just want to see if I can do it.  I know it will be difficult.  I plan on using a PIC to control the hardware.  I initially thought about using a PC as the brains, but started to think about the problems with Windows (being that I'm a windows developer) not being a real time OS, so I abandoned that idea.
I have been reading over various old school pin schematics and (very) basically the components needed are a Solenoid driver board that can drive about 10 or so solenoids.  I switch matrix to register the inputs.  A lamp matrix to light the thing up, and the motherboard.  Oh, and obviously the various components on the playfield.
I'm in the process of rigging up a test bench so I can begin development.  Once all the software is written I imagine the real difficulty is the physics involved to get a good playable playfield with very little dead spots.
I figure this will be a long project, but should be fun putting it together.  However, as was said by Tailgunner, would be easier to restore an older pin.