There have been some compression ignition engines manufactured for motorcycles and mopeds in the past.
the Lohmann moped with its 18cc diesel engine comes to mind.
http://cyclemaster.wordpress.com/pag...nn-hilfsmotor/
What seemed to work well was using 1 part pump diesel, 2 parts gasoline, and 1 part naphtha for the fuel (along with a 20:1 oil ratio for lubrication).
But then this is a 18cc engine, so its larger displacement would allow it to run without ether in the fuel.
I assume on a cold day, you would have to use starter fluid though.