Would adding Nitromethane in moderate proportions to your fuel mixture improve performance of an air-cooled engine?
What proportion would be best to use? and would this do any damage to the engine in the short/long term?

It would be expensive 4 sure

I used to run speedway cars and we would sometimes ad up to 3% nitro to our fuel. Will make a huge difference in horsepower and torque, just make sure your bottom end is pretty sound. Shouldnt hurt anything as long as you dont over rev. I would make sure to flush the engine with standard fuel after every run though cause it can be corrosive if left sitting in the chambers unburned. Also keep it rich as lean equals extreme heat. Go to a bigger jet size if possible.

For those unenlightened, nitro-methane is two stroke model airplane/car/boat fuel... readily available at all good hobby outlets.

I have some in my garage. :