Ok, about those "new" boards. All those 3 in one, 6 in one, 9, in one, and 12 in one boards are all bootlegs. Specifically they are slightly modified PC motherboards with JAMMA edge connectors on them. They all run MAME.
There are a FEW legit new classic boards, but they all only come in cabinets. The Centipede/Millipede/Missile Command/Bolwing one, the Ms. Pac/Galaga one, and the Qix/Space Invaders one.
It is very simple why they don't make replacement boards (beyond the obvious fact that the companies, and or distribution, support, and production facilities are gone for most of the copyright holders).
There is simply no REAL demand for any of it.
Bootleg Ms. Pac boards have been in production FOREVER, simply because there IS a demand for those. No demand for anything else, so no boards are made.
The setup cost simply to produce a run of boards is really high. I am sure some of the electronic genius's around here could implement (for example) Crystal Castles on a single board the size of an I-Pac with only a few chips on it. The problem is the fact that that their market would be MAYBE 100 people. Most classic games had production runs well under 5000, and realistically, there are a lot of games out there with dead boards, but there are not that many copies of each individual game with dead boards.
But, THERE could be a solution, it sort of already exists.
PC based JAMMA boards have beem coming out of Asia pretty steadily now for a while. Problem is that they are all set up as lamo 3 in one, etc deal with the worlds most common games on them.
Those boards are computers. Those boards can run anything. What I would LOVE to see is one of our (legitimate), boys making those things minus the game. Have them ship with two rom sockets on them, one for the OS (romdos probably), and emulator (Dmame). That one should be populated. Then leave an empty ROM socket that comprises the Mame "roms" folder, and let the buyer acquire his own rom. That would be a universal replacement JAMMA board that could be used for even the rarest boards.