The Israeli FAL I had used a bolt carrier with a fairly long slot cut in it, it was an all Izzy kit and a SBL receiver that I bought in the 80's. IIRC the dimensions that MuktukJoe has are right. The forward assist was similar in operation to the Portugese AR-10. The charging handle stays stationary until you need it. Then you retract the handle and push in on the knob. The knob is spring loaded and has a nub that extends into the groove of the bolt carrier. That allows you to manually force the carrier forward.
Obviously, there is some question as to if it's worth it or not. It can't hurt, since the rest of the time the charging handle just sits there, sorta like an AR's fwd asst. You can just ignore it if you don't need it. The Izzy knob is farther aft than the standard FAL knob. That is so it's over the bolt carrier cut when closed, so you have full travel closure using the forward assist feature. I found it to actually be in a more comfortable position and it's bigger and easier to use. TAPCO made some and some carriers to go with them, and those would count as a USA parts as well.