Thanks for the help! The wheel/ tire circumference would need to factor in--I also have some 700 rims but they are about the same size as the 27" if memory serves.
I called Bicycle-Engines.com and they do not carry just the sprocket in 40 tooth--she said only a 36, 44 or 48 as Damien1307 notes.
I probably used the wrong word in "tall", but I am interested more in a cruising gear (lower rpm-less teeth) as opposed
to the larger rear sprocket option. I just saw 40 tooth sprockets online somewhere else--haven't looked for a 38 tooth yet.
link:
https://www.amazon.com/Flying-Horse-Motorized-Bicycle-Sprocket/dp/B00KWLLG6O?th=1
Thanks again. John