Cushman / Predator / Jackshaft Questions

#1
Greetings
I've taken on a new project, I found a 1959 Cushman Highlander scooter and am doing sort of a Resto-Mod restoration on it. It came minus the engine. Since I can't afford $1000. for the original Husky motor, I'm going to go with a Predator 212 cc engine. I've used that engine with the Comet Torque converter on my Bar Stool Racer and my Doodle Bug builds.
This time I need to use a Jackshaft to get the final drive to the right side of the Cushman. The original rear sprocket is a 60 tooth.
How do I even begin to figure out the (2) sprockets I will need on the Jackshaft?
I know someone has done this before, and I'm sure there is some huge mathematical formula to calculate low end take off speed plus top end speed with the different gearing. I notice some of the Jackshaft kits sell you a 10 and 14 tooth gears for a mini bike jackshaft set-up.
Which one comes off the Torque Converter and which one goes to the rear hub 60 tooth?
Is it all trial and error, to get it dialed in? I'm not looking for a racer, just something that will do 30-40 MPH.

Thanks
Gadget Man
 
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