The standard H49 type sprocket will not fit an AM gearbox, which requires an M49 sprocket. This has a larger internal bore, with different splining, to suit the different mainshaft sleeve in the AM. There is also a deeper recess on the front face of the sprocket to allow the securing nut to fit properly.
The current RE price list on the Burton Bike Bits website shows the M49/20 as available, but I have not asked them directly.
The 16" wheel doesn't overly affect the gearing as the diameter over the fat tyre is about the same as that on the larger-rimmed wheels used on other models.
Incidentally, only some of the Apaches used the AM 'box, others had the 'standard' Albion.