From the FAQ:

The version you use will depend on your computer processor. In theory, the most up to date version of MAME you can use will result in more accurate the game emulation which usually required a faster computer processor to run the game at full speed. If you have a 64-bit processor then you want to use a 64-bit version of MAME (games can run up to 40% faster). It’s best to use the newest version of MAME as long as your computer processor can handle it. We recommend that you use the version of MAME that is offered on the ‘Download’ page. This special version has been modified to recognize Mortal Kombat 2 Plus (etc) as its own game. You may chose to use a different version of MAME, but you may get an error when trying to load MK2.ZIP. Don’t worry though, we will go over how to get around such problems in a later section.

Fightcade is the community-adopted way to play older fighting games online.

>Mortal Kombat 2+ is compatible with any version of MAME between 0.119 and 0.203.

It seems that it works in standar MAME builds, but they recommend their custom compilation for best performance.

> This needs a custom build of MAME to play?

My immediate thought as well, especially without any indication of source for either GPL compliance or just to know what they changed.

It’s because the hash of the roms changed so make complains