The larger motor, EM37, was introduced in the MY 2013. There are only two different transaxle ratios and they are very close so the motor is the thing to consider.
Other model years have variations on the batteries which may or may not be a concern for you. The motor/transaxle comes with a charger on board so it is preferred that you end up with the same battery arrangement if you wish to use that charger. Buying a whole LEAF is the way to go.
I need a transaxle part, so if you decide to use the motor with a different transaxle please let me know.
