cextion and I completed this today - it took us 2 days of work.
A couple of hiccups we had which were frankly minor given the scope of the operation:
1. At one point the game refused to boot, which was alarming, resolved by jumping two pins on the motherboard. Apparently this is a thing that happens sometimes?
2. We got trough errors which turned out to be one of the 4 pin power connections to the supply that wasn’t plugged in.
3. There are some dangling wires under the castle playfield causing stuck balls, so we have to pull the castle and sort that out.
4. One of the rubber stoppers that holds on the rainbow plastic broke during the operation. We improved a macguyer solution, but the kit really ought to include fresh ones for this possibility. (It does include fresh plastic locks, just no rubber ones.)
All in all this was a long but very rewarding project. I enjoy legos and things like that, this was fun to do and I am happy his WOZ will now give him years of trouble free use.,