Any ideas why they fitted the old style engine to the MD 90 & not the newer 12volt style that is fitted to the later Cubs? It seems odd that they would not use that newer engine & go to the trouble of modifying the old 6 volt style to 12 volt & cdi ignition. Maybe the 6 volt is a better engine?
Maybe MD 90s were made before the 12 volt engine was manufactured & it was cheaper to modify the engine than the MD frame, as the mounting holes are different. 
Just a thought that has been going round my mind in these idle (for some) times.