I lidke the Merlin plugs too. For use with a reed Cox engine, I would use 3 to 5 gaskets to control the compression. If the engine tends to stop and just bat back and forth, it is overcompressed. Otherwise, reduce the number of gaskets until it will only just run right. For short duration free-flight, you can use very high nitro. For CL stunt, I would not go over 15% nitro and a minimum of 20% oil, preferrably half castor, half synthetic.
Adapters for conventional plugs are fine for sport flying, but not for competition use. They cost power and reliability. The problem is that the shape of the combustion chamber is totally ruined by the position of the plug and the threads that are exposed to the combusion chamber. Also those plugs seat at the top of the threads, so who knows what the compression ratio is? By substituting a Merlin Glowbutton for the regular plug in a Brodak MK 1 .049, I gained 1500 rpm and the run was MUCH more stable. The Nelson head added ANOTHER 1500 rpm beyond that! These changes turned a casual sport engine into a competitve tiger!
My standard setup for 1cc stunt is the AP Wasp .061 engine, modified with the Jan Holuszko (do a search for his contact info) venturi and Merlin glow button with 1 or more extra gaskets, depending on the Nitro and prop I am using. Other than that, the engine is totally stock and turns about 21000rpm on the ground with an APC 5.5x2.5 prop. My new model, the Sky Streak (search again) is actually going too fast on 48' lines. I will cut the Nitro down to 7.5%, and may try the 6x2 prop to make the speed liveable.