More options are a good thing. The "H" is the height of the magnet (its height if a hubmotor is laid on its side, don't ask me why).
If we are talking about a motorcycle kit that has the motor mounted cross-wise, with a chain/sprocket with one run between motor and wheel (which is the common orientation)...we might call the H-number the width of the motor stator in millimeters.
The 138/165 is the diameter of the cylindrical stator in mm. In some applications, the larger diameter motor will fit, and if it does, it will make more torque from the same amount of input watts.
You want to start any conversion with a motor that is as large as possible. Once the diameter and length are maximized to fit, then you can determine how many watts can be fed to it.
The factory 2:1 gear reduction is also very valuable, because without it, you may be forced to have one of those huge sprockets on the rear wheel. That wouldn't be horrible, but when it comes to replacing a worn sprocket or changing the tooth-count...a "normal sized" sprocket provides the best selection and lowest prices.