Are you running front hubs or using direct spindle mount wheels? If you aren’t using hubs currently you can achieve what you want a lot easier and probably cheaper with a set of 17mm hubs and hub mount front wheels.
The Easykart is just a Birel R30 though and any of the newer style Birel spindles are heftier and give you additional width.
If you are really set on changing spindles though here is a page from Comet Kart Sales showing the different Freeline options. You have #1, the #3 is an easy swap and still has a 17mm shaft to accommodate your current wheels or hubs. The #5 is a nice piece but will require new 22mm hubs as well as the spindles.