Solution found. I picked up another Briggs 3hp for $20 which has a 5/8 shaft. Swapped the engines over (I'm getting pretty quick at this with all the practice) and fit the pulley. Fit great and sat square as I'd hoped. The only trouble now is the A27 v belt is too short and I will need to get a 29 or 30 (once I measure up and work out which will fit best). Also, the other Briggs can now go back on the go-kart. Changed the plug in the new motor and it ran well. Probably just needs adjustment of the carb to smooth things out. I'll wait until I get the belt and test it under load. [Linked Image]
[Linked Image]