Matador Models have come to our rescue and now for the princely sum of 90p (about 1.30 euros) make an aftermarket set of five Hotchkiss and Lewis machine guns suitable for WWI tanks
They also now do a 3 inch 20 cwt AA gun on the 1940 platform - hopefully they'll do a WWI version either seperately on a pedestal or on a Peerless lorry
I am myself building (slowly - lack of time) Matadors recently issued Tadpole-tail conversion for the EMHAR Mk IV. Also: they have picked up SMA's really excellent Italian WW1 149mm Howitzer kit. So their Great War-comittment continues.
The Arizona Models folks here make 1/12 scale mg kits up through full scale for use on RC and replica aircraft. They also say that they do custom scales.
I have never ordered from them but I have downloaded their assembly manuals and blueprints. These are free on their website and provide a wealth of information that is useful for scratchbuilding mg's.