The Specna M249 range is a direct copy (potentially just rebadged) A&K. If you look on the gearbox casing it should have an A&K marking.
Regardless, the gearbox takes standard V2 gears and IIRC the motor is a short shaft. When you say you want to improve RoF, by how much? The box mags aren't great at keeping up with ridiculous RoF anyway but more to the point, it's a support gun. You can suppress better with a lower rate of fire.
As for range - better hop rubber is where I'd start. Most of the aftermarket hop units seem to cause feeding issues, if you're not getting enough hop it could be suffering from the same problem that the Classic Army and A&K guns do that the bit that presses down on the nub just doesn't have enough travel. Google for "A&K M249 biro mod".