I was about to buy a pair of aftermarket WWI German machine guns (Spandaus or Parabellums: the sort of thing you'd see on the cowling of a fighter) for a wacky project, but it then struck me that a serious WWI modeller might have already done this and have the "inferior" kit parts spare.
So, do you have a pair of MGs going spare? If so, what do you want for them? I'll pay or swap - up to you. Bear in mind that the aftermarket ones are £6 + postage, so postage charges will probably make doing a deal outside the UK non-viable, unfortunately.