I do believe systema makes mosfet-based switches, but its cheaper to make your own:
http://www.airsoftmechanics.com/guides.php?aid=14
Failing that use a relay of appropriate amperage from a vehicle.
Reference turning AEG into semi-only, that sort of defeats the purpose of
Auto Electric Gun. Simply prevent fire selector switch from going onto 'auto' position? A screw to limit its turnability?
Argh mcguyver, you must have an extra typing arm...