Hey guys I'm working on a custom MP5 project right now. Right now it's a TM MP5A4 with an added front rail system and internals upgraded through AEX to shoot 375FPS.
You should have no problem with adding a longer barrel, maybe replace your bucking but thats outside the gearbox. So no, you will be fine, just dont get too long of one. Also you can refer to mechbox.com and good old fashion youtube.
K.W.A. ATP
Echo 1's new SOB-1 utilizes an M4 stock. You could probably contact them in order to purchase one alone, as well as ask them about fitment into your marui.
Custom PGC/Guarder TM G18c