I saw where you mentioned using a 72t rear wheel sprocket on, what I think you mean, was a Coleman CT200 mini bike. I've been looking for something like this for better steep-trail hill climbing. Was it the Coleman, & if so, where did you get the sprocket? I'm using the factory jackshaft arrangement 10->20/<?>->50t rear (#420 chain).
monsterscooterparts.com gives the rear sprocket specs as: center hole dia: 1-9/16", mounting hole dist: 3-1/8".
-Howard (thirtyforty, also email@example.com)