CNC Machine. It seems like a m4 internal might fit. Try to fit a v3. I don`t know how you would get the polymer parts. I would just CNC it all out but expect it to be like 25 pounds and cost 500 dollars. Just saying you will have a VERY CUSTOM GUN.
Edit: To Olle. Haha I didn`t see the bolt. Well try to fit a L96 internals in there.
It would be doable without a whole ton of CNCing... if you are willing to work with polymers and make some compromise on how accurate it is to the real thing...
If I were dead set on making this here is generally how I would go about it.
1. Start with an AEG AR lower that I like
2. Build a chassis to fit into it that would be able to:
a. Hold the bolt and sears into the body
b. Mount to the two pin holes that would normally hold the upper.
c. Extend out for the fore grip attachment point
d. Mount the outer barrel to
e. Mount a G36 or UMP folding stock to.
This part would be rather minimal, but fairly complex. if you are creative and think it through you could probably do it in one part and one trip to a local machinist.
2. Make a wooden mold for the "upper" and set it up on a vacuum table. Do it in 2 parts split along the fore grip, probably out of ABS
3. Find a 22" RIS rail and modify it to mount along the top of the top half and epoxy the two halves together so it comes off like a G36 fore grip plus a few extra screws that attach it to the chasi.
Once I attached the UMP/G36 folding stock thats pretty easy to modify to look close to that.
Simplistically following this you could get pretty close and use essentially the 'drive train' of your choice to power it. The only aesthetic things that you couldn't easily duplicate would be the mag release, the bolt handle and the added length to the back of the receiver. The other stuff is pretty easy, takes some practice, but if you have a shop vac there are really really easy ways to make vacuum tables; and for a part that small you could use your kitchen oven...
Personally though I'm more a fan of Desert Tactical boltys