On about four occasions I have had difficulty in starting Scotts with rebuilt engines or that have been standing for a long time. It seems to be a lack of suction in the crankcase caused by the glands not sealing adequately. I have managed to get them started by a push start as the increased speed of turning over the engine negates the leakage past the seals.
On one occasion I had to resort to a long bump start down a 1/2 mile hill with the engine popping and banging until the engine would run consistently. 15 minutes later after riding it around and back up the hill it started just the way it should. After a bit of regular use the bike would start from cold on 4th or 5th kick. This was my TT Rep I had not used for about 6 years.
My 2 Speeder, after its rebuild, also needed a good push around the garden before it started for the first time. This bike seems to like full advance when starting from cold, possibly due to the spark being a little weak on retard or a cold mixture being reluctant to fire. When warm it will start first kick and must have the ignition retarded otherwise it will kick back.