Quoted from HHaase:Since it was a few games into the WPC era before they started using the fliptronics system there's a good chance the patents are still in effect on the flipper boards. Same with many WPC generation parts. Also have to watch out when laying out a board, as trace layouts actually fall under copyright and not patent, and thus are valid for a much much longer time.
Pretty much all the pre-Pin2k patents have expired at this point. Copyright is of course a different story (nothing post-Mickey is ever likely to expire again) but unless he literally photocopied the board layouts that wouldn't be an issue.
Too bad he's insisting on going full crank mode on the other stuff though, I think the Thunderbirds would be a pretty cool theme.