Hello Guilherme
I presumed the Fox 50 would be a good stunt motor so have bought one to try off Ebay but I've not put it on  a model yet.
It is based on the 40 heavy weight C frame case, you get more displacement but it is still heavy compared to other modern 50-56 size engines and has a wide spacing on the crankcase . They had a reputation for pre-ignition problems when used as a R/C engine and the head had its chamber redesigned a few times to help solve this tendency.
I believe Fox only made one size (8mm) control line venturi that was shared with 40-74 size engines with the bolt on flange intake which normally means a custom set up is needed (7mm may be a better start). There were some Fox engines with a Perry carb adapter and this would be easier to work with when setting up a control line venturi.
Regards Gerald

     Can somebody suggest a method of converting this to CL other than wiring the throttle open?

   

Hello Frank
I have flown many a large Fox engines with its RC carb set open and must say they are often better then the Fox venturi as supplied .
Don't think MECOA has any C/L venturi's left but you may want to contact them to verify this.
Seems the best set up will involve some metal work making a flange and welding on a tube adapter for a separate venturi or search E bay for second hand .
If you like playing with JB weld you can get very creative here too. (I may post a separate post on this topic)
Regards Gerald