If you're just going to be using 7.4v LiPo then a MOSFET isn't required, your trigger contacts won't wear down any quicker than they would with an 8.4v NiMh. However, you'll see benefits in rate of fire and trigger response if you do fit one. The UMP is pretty tight for space so I'd recommend something small, the Gate PICO SSR is pretty tiny and will do the job nicely : http://www.fire-support.co.uk/product/gate-electronics-mosfet-switch-picossr-2
You'll need to take your gearbox to bits to fit it properly though, not sure if that's an issue but if it is there are plenty of people round your neck of the woods who will do it for a reasonable fee.