I got the jackshaft assembly from BMI..I had to cut the center tube out so I could move the drive gear to the inside for my purpose in able to get the drive sprocket aligned with my rear wheel sprocket.. I added two extra bearings after cutting the tube out.. Just bought the extra bearings for the inside from Aces Hardware.
What is nice about it is it just bolts right in the motor and has the motor bolt pattern..It seem pretty stout, and is a pretty easy way to accomplish the task..They sell just the plate without a shaft too..
They sell other I am sure higher quality jackshafts too.. But I am kinda cheap, I always buy cheap and fix my errors later.. But seriously I think in this case it will suffice nicely..
Forgot..I bought the slotted mounting plate from them too.. I could have made one in the mill..But for what it cost..It was not worth it..
Bought it but they are on Amazon too..Thanks
https://www.bmikarts.com/Clutch-Mount-with-Jackshaft_p_1129.html