You might try Henry Werner at Virginia Hobby Sport.  I got a couple Fox .049's off him last year. He may have a solution.

I am not an expert by any stretch.  I have 2 Fox .049's 1-an FAI version and 1-Sport with FAI crankcase (nostalgia legal).  I have bench run both and flown the sport one on a Musciano/Blackhawk Flightmaster.  As far as the numbers I don't have data.  Next time I run them I will document and post.  I my seat of the pants feeling is that while they are still breaking in they run much better than a reed cox but not up to a potent (modified with, pressure back plate, cylinder shimmed KK needle and bored out venturi, Globee head) TD.  Time and break in will tell.  I feel there is more in them but they need more running.  I have run them with Sig 10-20 Powermaster 15-18 with added castor and Sig Champion.  My overall opinion is that they have good potential and are a well made engine.  I got mine from Henry Werner at VA Hobbies.  They are assembled from NOS Fox parts by Doug Martin and have been run prior to shipping,