I agree on the front wheel, 21 will work but not as well as most any other size. Going to a smaller wheel also some what helps lower the overall trail making for lighter steering. Triple tree's are no problem for this bike. Reverse can be added to the bike but you need pipes going down both sides of the bike as if they are both on the left side the pipe for the rear head is in the way of the gear shift lever. Not sure if Liberty or Champion has mounts for your bike, we can provide them for the Champion however if you buy the Champion direct from Champion and live in California you will have to pay sales tax, if you buy one of our sidecar and we ship it out of state, no sales tax.
Last I heard Liberty's lead time is around a year, Champion is at 6 weeks, we are at 10 weeks.
Let me know if you have any specific questions.