Thanks to a German collector I've been able to put my hands on a System 24 boardset.
Better than that he actually sold me a motherboard, a romboard, a floppy controller and floppy drive!
I'm now fully equipped.
After building a System 18/24 adapter to JAMMA and hooking the board to a video converter (remember it's 24kHz) I was finally able to test my work :
All my decrypted games work ! Except for Rough Racer which got stuck on the loading screen (not sure it's an error in the code or a problem with the floppy emulator).
I still need to test my ROMs to floppy conversions. I've also designed a Multi game kit (more information about that to come).
Rough Racer and Hot Rod require an additional I/O board (for analog steering wheels) ref. 834-6510 in order to boot past the warning screen. Rough Racer has been reported working by a collector having the dedicated I/O board. Thanks to him for his feedback.