Look around google for cheats, there are a couple of big cheat.dats for mame that have the cheats for all the MK games, they are pretty common.

What do you mean by hacking roms? Like getting in and changing graphics and stuff like that?

yes, exactly

You could use a hex editor to try to change some stuff by random but its not easy.

By the way does anyone know if there is somekind of tool or programm that can read out rom data an can show the graphics maybe?

I already have some tool that can read the whole music and sound effects from MAME games.

But graphics would be much more interisting.

there has to be some way.. I remember seeing some for the Goldeneye 007 Rom for the Project 64 EMU.. It was pretty interesting, u could edit the skins in the game and everything