Funny, there is actually somebody on Ebay selling a broken authentic Metal Slug X MVS cart, but the seller is from Mexico. You can try your luck there.
https://wiki.neogeodev.org/index.php?title=Repairing_Metal_Slug_X
What's the plan? Replace the Altera Max chip or transplant the ROMs to a BK1. Or did you have another idea?
Funny enough, that is exactly who I bought it from yesterday
Nice! Good luck fixing it and let us know if you did.