G&G Upper Receiver Replacement

[Khaosbonk]

I have a G&G MP5a4 that I'm looking to upgrade the receiver to a metal one whilst keeping the blowback functioning.
The problem is that it's the plastic/nylon fibre, Pneumatic blowback version and I am not sure what metal upper (if any) would work without a fair of fitting.

Anyone have any advice or played around with something like this before?