Yeah I know but I kind of like it, although I can understand why it never made it into production, and I'd like to have some rare games in my collection. Scramble is not a bad game either but I get sick of it easy, and again I like rare stuff, and aside from that I'd like to gradually create a FAQ of board conversions that are confirmed to work and how to do them, Speed Coin is one of many I'd like to try.
What I've initially tried was a straight ROM swap using the MAME ROMs and going by their names for placement, my Scramble board isn't working right but with the original ROMs it ran enough that I could see it working in attract mode, with the Speed Coin ones it only seemed to show gibberish. BUT obviously a malfunctioning board isn't the best test bed for this kind of experiment, so I need to fix that first before I write this off as a failure, and I haven't swapped the color PROM yet so I need to try that too before I give it up and move on.
BTW did your conversion work? And did you need that stupid external Konami security sub-board with it or can you just plug a Z80 processor in its place? I have a spare Super Cobra board around but it doesn't have that board, I was going to write that one off as just for parts but if it'll do Speed Coin with just plugging in a standard processor then all the better!
Other stuff on my rather ambitious and probably delusional to-try list:
- Venture to Teeter Torture conversion / have all the parts lined up now, just need to wire a harness and confirm the original boards work, then give it a whirl. If successful design side art, bezel, marquee and build it a cab
- Figure out how to get around protection PAL on Sente ROM cards as to convert them to like Snake Pit, Stompin, etc

- Double Dragon to Dangerous Dungeons conversion / I'm very doubtful on this as I'm pretty sure the later's kit needed a security chip
- Get one of the two millions sets of Victory boards I have working then design it a mini-cab
- Looping to Sky Bumper conversion / pending no protection devices they're so simular they must use the same boardset
- Road Blasters to Marble Madness conversion / supposedly both games use the same Slipstick protection chip, I plan to find out!
- Converting Spectar to Side Trak by building it's sound board from scratch, which isn't particularly complicated
- Circus to Rip Cord conversion / have all the parts, same deal as the Teeter Torture experiment except I'll have to make PROM to EPROM adaptor sockets
- Figure out and design a solid state conversion kit to substitute for the crappy cassettes of DECO boards
- Qix to raster Taito version of Black Widow / supposedly they used the same board, just anxiously waiting for the ROMs to turn up!