RE: Difference between OS 46 fx, and fxi?
O.S. made a change to the FX line during the life of its production. Later FX series engines used the same kind of seperate bracket that the FXi did.
The remote needle valve bracket that was integrated to the backplate had another problem besides being expensive to replace. Because the bracket was integrated with the backplate, it acted as a heat sink and transferred heat to the high speed needle after the engine warmed up. The backplate and remote needle bracket were redesigned more to prevent fuel vaporizing, reducing repair costs was simply an additional benefit.
I believe I read this explanation in a Glow Engines forum post, and it was a reply from BAX if I recall, so I'm confident that this is correct.