Has anyone done this; my brother had an old AMF Harley trail bike. It used two sprockets on the rear and a spring tensioner. You had to switch the gear manually. I was thinking that would work for these bikes. Have a 44 tooth sprocket and a spacer then a 36 tooth sprocket. The chain tensioner would need to be a bit wider and with lots of swing to allow the switching of gears. Windy days 44, calm and flat roads 36.
Always do a pre-ride check. Make it a habit, and ride safe.