With electric RC etc there is not alot of 1/2A new engines around. Cox engines can be found allmost at any collecto/ club events/ ebay. The main problem is plugs and they need to be clean. There are several makers of plugs. RJL/ Doug Galbreath/ glow Bee had some etc.
Folks these were made to run on nitro 25% is a good start. Save your plugs and use head gaskets. The reeds need to be in good shape and if using TD's clean those tiny holes out in the Venturi. A pin does job quite well. You need to use castor oil fuel. RC type fuel need adding castor. Sig sells it and every hobby Shop carries it. Even Super Markets carry Baker's castor oil.
Dan