Re: hi im new
I'm not real up on bicycle stuff as far as frame sizes, but I believe I read that you will need at least a 22" diamond frame type to mount a bicycle motor kit on. Is that what you mean? I know it will fit most standard cruiser types as long as the tubing is the older standard (not oversized) tubing. I mounted one on an old single speed Ross Europa, with room to spare. (diamond frame type, single speed).
And yes, the new Moon Dog is slightly different than the old Moon Dog. The frame looks different (differing bends) and different color. I haven't check them side by side, so there may be other differences that I am not aware of.
If it helps, these bicycle motor kits were originally designed for the older diamond frames, 1950-70's. As for fitting one to a newer bike, where there's a will, there's a way.