A couple things, since I recently converted a NEO GEO cab to MAME, with an ArcadeVGA card:
-Like tetsu says, a JPAC doesn't replace the need for an ArcadeVGA card
-I'm not sure if the JPac works with Jamma+ without some slight mods.
-Checkout
www.hardmvs.com for info on NEO GEO, Jamma+, etc.
-Get your monitor working with the NEO GEO boards BEFORE you start switching everything to MAME. This will eliminate a lot of the guesswork.
-To get your NEO GEO boards working...are you sure the power supply doesn't work? You can quite easily use an old PC power supply to do the job if need be. Again...check out hardmvs.com for the wiring
-To power your monitor....you should have a big black transformer in your cab. This is your isolation transformer. The output from this should run to your AC inputs on your monitor AND NOTHING ELSE.
I didn't use a JPac, so I don't know much about it. It isn't too hard to put your IPac to use though.
To hook your ArcadeVGA to your monitor (without a JPAc): ultimarc also sells an VGA signal booster, which acts like a nice breakout, with the added benefit of having +5V, which is what NEO GEO boards output (as opposed to the 1V coming off the ArcadeVGA card). Many people say this isn't necessary, but I'm lazy, and really like the clean screw-down breakout of the amp.