The centre target bank (a set of three standups that when completed are lowered beneath the playfield to expose a centre ramp) hasn't been working for a little while, and I finally got around to looking at this tonight.
As is typical, it was simply a broken wire - which was stopping the switches from registering. A quick solder job and we are back fully operational.
Of course, with the new ROMs all the high scores are reset. I have put most settings on hard but managed one high score so far. Nowhere near my best, though.
It's nice to have it back (almost) fully working. I have one thing left to troubleshoot - the MILLION spot letters do not light up, nor does the Build Jackpot light. These lights are right at the end of the playfield and will require lifting the playfield right out in order to get to them for a proper look. As they are not that important I will leave this for later.
This pin is so addictive - really has that 'one more time' feel to it. (Thanks Dave and Zivko!)