Due to no one else responding I will put in my small observations.
Mark Goodson's 630 or so Comet is a joy to ride, this from a man who is not enamored of Comets. It goes not a lot worse than my Rapide.
The engine done by Bob Dun, is bored and stroked 90mm by 100mm I think.
Bob tells me that it is the longer stroke that makes them go.
I Too Have A Bored And Stroked Comet Converted By Bob Dunn.
If You Cant Afford A Twin This Is Definetley The Way To Go. The Main Difference Is The Installation Of An Amc/norton Gear Box As Bob Said The Burman Gear Box Can Be A Little Too Femmer For The Job. You Would Be Better Off Speaking To Bob About The Technicalities As These Are Way Beyond The Likes Of Me
Have been busy catching up from 5 days off due to going to the Dutch.
Neil has just about got it right. The only bit that he left off is that you need to machine a squish band on the piston and in the head.
In my view Bob Dunn is the man to do the job and I am just sorting out a crankcase, head & 11mm shorter rod for him to build the big engine that is going into the Vinparts trike