That's a really nice looking engine, Brian!! Do you run a "single side" jackshaft (both pulley and clutch on same side) since the engine PTO is already inboard? Or do you run a belt driver on the flywheel side to the j/s? If you took that flywheel retaining shaft, turned it to 3/4" and milled a small keyway in it, that would be a really slick stub for a 3/4" bore 8mm belt driver...
You can do that on a mild built, but you'll break the crank if the engine's got any testosterone.
I have a 3/4" crank here if you choose to go that route.
The jackshaft is kind of a hodgepodge.
The one on my kart works outboard like a 2 cycle.
The Burris jackshaft & big block mount won't work as it comes on most karts. I just went through this with a 390 customer -- I think he ended up getting a shaft from Kermit to use in his Burris j/s kit. That way he has keyway rather than spline shaft, and keeps both the driven and clutch to the left side.
Nothing drives off the flywheel side. I just had the cover off to change the coil in this picture.
Belt drive can be done with a belt drive conversion on the clutch. The crank is PLENTY long and can be cut shorter if need be - some large seats require it.
I've got another road race customer going with a belt drive clutch on his animal engine.
There is nothing "off the shelf" about this as every kart and engine/clutch combo is different.
I "think" that's why guys will buy "kit" 460 or Vegas engine packages, so they don't have to do all the homework and source all the parts (which can be a real pain, especially with this covid mess.)