Two main types of basic mosfet - active breaking and non-breaking.
Non-breaking is the simplest and controls the power flow to the motor using the trigger contacts as the signal.
Active breaking (AB) essentially puts the motor into reverse momentarily to slow it down quicker. This produces a more consistent firing cycle but will produce gradual degradation of the motor due to carbon build up and will heat up both the motor and fet to some degree (the kinetic energy is turned to heat when it breaks the motor).
I tend to only use AB when I'm having bad overspin problems which can cause double firing on semi.
If you want a basic FET then the Gate Pico SSR3 (non-AB) and Pico AAB (AB) are very small, relatively cheap and reliable.
If you want to get extra functionality such as burst modes, pre-cocking, adjustable AB etc then a good option is the Jefftron programmable unit (this replaces you trigger contact mechanism - see my post about it).
If you've never done any soldering or electronics or not taken a gearbox apart then I would think about buying a cheap second hand gearbox to practice with before messing with your main AEG - you are almost certainly going make a mess.