You can get a nunchuck battery to fit in the foregrips and you wouldnt need to replace the stock.. I have a 1700 mah, not sure if its a nunchuck. I have a 9.6 1700 mah butterfly battery and it fits in the foregrip of my m4. Oh, i may have forgot to mention that the C8 has a different barrel than normal m4a1's. You know how there is ridges on the M4 barrels to mount m203's? On the C8 the barrel has no ridges as you can see here:
Canadian:
http://www.sfu.ca/casr/101-c8a1.htm
American:
http://www.colt.com/law/images/m4.jpg
I am not exactly sure where to get the barrels but i know G&G makes a Gr16 A2 carbine which is an m4 that comes with the straight barrel.
Edit: You could also put a ready mag system on it for a large battery. I am not exacly sure if this is only used by the US forces only though.