Get a pingle airshifter for a Harley off of ebay (got my last one for $200 complete). Works great and way cheaper than Sharpshifter (I have both). The Pingel woorks with a very small battery powered compressor that will last for a typical road race. One problem with the axle compressors is they make no air when kart is stopped oin pits and sometimes not much at slow pit road speeds. I’ve ocme to prefer the battery compressor.
Pingel also makes an all electric shifter w/ ignition interupt which may be a better option.