You'll get lots of suggestions on this one. The top three will be an ST-style NVA (you can get new ones from two or three vendors here -- at the risk of leaving someone out I'd check with Randy Smith, Jim Lee, and Stunthanger Hobbies), an OS LA-25 NVA (Tower Hobbies), or go see if Fox has one for their CL version of the same engine (check the Fox website).
The ST, at 0.156" diameter, may be a bit big for your Fox engine. I think my first choice would be to see if Fox made a CL version of that engine, then see if they'll sell you a needle valve assembly.