If you want it for display only, no probs at all. You have to file the mag catch hole making it larger on top. I would recommend taking the mag catch lever out of your Marushin M16, then you will see. Then reinstall it and test-fit. It is all about filing and test-fitting. You remove too much, and your mag will rattle. I also noticed that real mags as well as Marushin models differ slightly, so one mag fitting one Marushin model does not mean it will fit another Marushin model too.
Making it feed properly is very difficult as the feed lips have to be reworked. I did not yet manage to do that properly. The original follower will work (the current green ones seem to be the best), but the spring is far too stiff. I have yeard some are using the Marushin spring from the 20 round magazines with good sucess.