Hello, I'm new to this and have a quick question.

[Lasher2200]

So me and my friend have an opportunity to buy an old Metal Slug 2 arcade cabinet and we plan on putting a Multi MSV cartridge on it if that's possible. We've never done this before so I'm trying to learn. I think I got most of what I need to do down. The one thing I need help with is converting this from three buttons to four buttons. How would I go about doing that and about how difficult would it be?

[Slippyblade]

Depends on if it's a dedicated Metal Slug board or if it's already an MVS motherboard in there.  Take a look at the board and see if there is a cartridge, about the size of an old VHS.  If there is a cartridge, then you've got an MVS mobo and what you want to do is dead simple.  The wire harness probably already has the wires for the fourth button and you'd just need to drill a new hole and wire the button.

If there is no cart, then you've got a dedicated PCB, in which case you'd need to acquire a one slot mobo in addition to the multi-cart...