Are you trying to achieve a specific look? If not the best bet for a metal body for a G&G gun is the G&G metal bodies I linked to. As jcheeseright said the blowback gearbox in your gun isn't necessarily going to fit into another manufacturers body.
I know it's very easy for us all to sit here and tell you to spend more than you really want to but honestly - learn from our mistakes. We've already spent more time, effort and money than strictly speaking necessary to get where you want to so save your cash and either get a different gun or buy the right parts in the first place. It's not the big parts that catch you out it's things like - does the new receiver come with a charging handle? If not, will the old charging handle fit properly? If it does, will the new charging handle clear the gearbox properly? Will the wiring still fit where it needs to? Is the hop unit compatible with the receiver (rare that you find one that isn't to be fair but it happens). It WILL NOT be as straightforward as it seems on the surface.
All that said - the things I've learned throughout the experience will serve me well as I continue to airsoft and no doubt get badgered to fix my mates guns!