The Drone Engine was the gun engine to have for competition when they first came out. This pre dated the arrival of glows by only 18 months or so.
The Gold head plain bearing engines suffered with some shaft breakages and this brought the second series BB Drone engines out. These are a good performing engine but bear in mind you want to swing fairly large props with them.
As soon as the glow plug arrived, it sort of spelled the end of the dominance of the Drone as a competition engine.