One possibility would be to add wood.
Airsoft sellers offer real wood kits for replicas (which usually have wood-coloured plastic grips) which could be modified and included. Or perhaps you may look for some army surplus and look for an M1-carbine stock.
Otherwise I would revvomend a better pistol grip and some metal folding stock.
A plug for a bayonet may be an interesting add-on.
Cool Project Larry! 8^)
Here's a link that I hope may interest - http://www.hobbytalk.com/bbs1/showthread.php?t=161666
It gives an overview of a realistic metal effect for a model weapon from Stargate SG-1 ... Hopefully it can be adapted for your project mate!
I've finished modding my gun. Started by removing the stickers. Then I painted it all with glossy black Rust-oleum paint. I followed up with painting a faux wood look on the handles using a 5-1 mix of acrylic glaze and acrylic brown paint. This along with the black base and the bristles of the brush helped to give the look of grain to the "wood." Oddly enough the faux wood almost feels like wood when you hold the gun. I also painted the foam bullets with brass paint.
There's still a few things I need to do. I keep finding spots I overlooked with the black that I need to touch up. Plus, I'm considering painting a sealant on the faux wood and the bullets. Finally, I've bought a black pen laser pointer that I'm considering mounting on the side of the front handle. That's going to present a challenge and I'm looking at some options on that.
I'm going to video tape the gun in action and I'll upload that.
Overall, I'm very pleased. For comparison I've include the image of a real Tommy Gun that I used as a guide.