Yes it is more than possible to convert an arcade cab to be a MAME machine.
You would remove the Jamma harness and Power supply out, wire the controls to some kind of encoder board,
a I-pac, Cthulhu, Cerberus or what ever
Using the original CRT tube?
Ultimarc has the Arcade VGA, VGA video card that support 15 and 25 khz RGB Monitors.
You will still need to wire a pin adapter for the monitor it self.
PS: they also have a cabinet with a non working tube. will it be possible to just replace it with a LED/LCD monitor?
Yes, but be warned finding a low lag monitor will be difficult.
If you get the Metal Slug 4 cab what you are doing with the board and Metal Slug 4 MVS cart?