I made a System 1 to Jamma adaptor for testing. It just does power and video.
Because the system 1 main board has so many connectors, I don't know that it would be practical for a cabinet. Also, the system 1 main board + cartridge together won't fit in a lot of cabinets.
However, if you want to do it, here's the conversion to Jamma for power.
+15v ==> +12v
-15v ==v -5v
(both of these are for audio only so the different voltages only affect the power to the sound and are not a problem)
+10.3v unregulated ==> +12v
(this is needed to get the CPU to run)
+5vdc and ground are hooked to +5vdc and ground
.
You can connect the +sense to +5v and the -sense to ground (or leave them disconnected). The sense lines were for the atari power board to get feedback and adjust the power to the gameboard.
Good luck,
Rocky