Hi X-Bird, thank you for your comments.
I lack the vocabulary to fully understand your comments.
Your method 1: " You could shift the engine back or upwards, move the axle sprocket inline with the driven pulley and chain direct that way"
Do you mean eliminate the jack shaft completely and mount the engine next to the sprocket on the wheel? If that's the case, yes, I'd have to raise the engine up so the torque converter cleared the back wheel. I think I can manage that. I wouldn't have the same low-end torque because I would be eliminating the 20:12 ratio that the jackshaft offers, but maybe the torque converter itself doesn't need it (I had the jack shaft with a centrifugal clutch originally).
Your method 2: "could get rid of the driven pulley mount, raise the jackshaft and put the driven pulley on the jackshaft"
What is the driven pulley mount? Which is the driven pulley?
I totally get the disadvantage of a short chain. I'm using #35 chain, the cheapest I could find; from the pictures you see it is well oiled (because it splatters everywhere). BTW, both chains wear out fast. I will look into the HD chain.
I was thinking I could get this tensioner to fit:
https://www.ebay.com/i/251787455501?chn=ps&dispItem=1
I could mount it to that plate that the engine sits on, and then use a long spring to somehow pull it into the chain to the back of the go cart; always pushing it onto the sprocket from the bottom side of the chain.
The size of the engine is the Predator 6.5 HP.
Thanks again for your suggestions; any more would be appreciated.