Why run a shifter when you can run a Tag kart? Tags are only a few miles an hour slower and a few tenths slower on the track. Cost is dramatically reduced from both the buy in stand point and the maintenance required. You are spending less time messing with your motor and more time focusing on setup and driving skills which makes for a more enjoyable day. If you don’t have a good mechanic then you might as well just throw the shifter on the track for practice only because you won’t be up front.
Tag is the way to go. Buy a Sonik, Rotax, Leopard, Easykart or Biland. Several motor packages to choose from and they start in the low $2,000’s for brand new.