You want to use an OS F or a YS 4 plug, trust me on this. I had inadvertently put a McCoy into one of my 1.40L engines and wound up sending it in for service because it would not tune and I figured the bearings or ring were shot....Richard changed the plug and sent it back to me...problems fixed.

If you want to add a little more prop weight put an APC on the engine. 2100 RPM is still acceptable for idle and you are only using 20% nitro fuel, it would be easier to get reliable lower idle with more nitro.