Baja Reaction 150 Motor Swap with GY6150

My sons were given this project buggy from someone who decided they didn't want it any more. The motor was shot as it had thrown a rod and cracked up the housing. The buggy had an automatic reverse and a dual chain sprocket drive. The boys bought a GY6150 motor to replace it, with a manual reverse: https://www.ebay.com/itm/150cc-GY6-ATV-Go-Kart-Engine-Motor-AutoTransmission-Build-in-Reverse-Short-Case-/274621621422?nma=true&si=P29iAdyKT14RdCxVU1LwabCPWmk%253D&orig_cvip=true&nordt=true&rt=nc&_trksid=p2047675.l2557

The question is how to go from the shaft on this to the dual sprocket on the axle. The new motor shaft is shorter because there isn't the planetary gear set for the automatic reverse. I was hoping to do this as easily as possible, but would rather not replace the shaft.

Has anyone done this kind of a replacement before?

Pics

I attached some photos to help explain the situation.

The old shaft\sprocket\components are incompatible with the new shaft. I need an easy way to attach sprockets to the new shaft in proper alignment. I'd like to think it could be done without the use of canabilizing\modifying the old parts and\or using a welder.

First thing, you dont have to run a double row chain. You can space out the sprocket from the hub.

That engine is set up for a quad. Try finding a sprocket on that dealer's site. You may still need to shim the axle sprocket to get it to line up.

Or machine a splined coupler to slip over the output shaft with a center locking nut and will hold the sprockets on the outside of the coupler