I think I'm done with this. It really doesn't look much like an M113.
None of the smaller M60 class MGs that I had looked great, so I went with the 0.50 cal from the M151 kit. The ammo box is from the Italeri M998 IED kit. I added a couple of plates (also from the M998 kit) to provide some protection while loading the TOW and MG ammo, although loading a .50 cal even in a lowered position would not be easy:
The smoke grenades are from another M151 kit that didn't get used in anything before now:
I found a plastic bit that looked like the GPMG mount that apophenia posted, but it's barely visible in the pictures:
The bag for the spent shell casings was made from Procreate 2 part epoxy. It's great stuff. I'm not sure that the bag would be big enough to hold all the casings, though:
Rear view:
Thanks for all the comments, and a big thanks to Jeff for sending me the Lav-25 kit and Bill who sent me a partial M113 kit years ago; most of the parts were used.