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.

http://www.cometkartsales.com/store/chassis/birel/images/birel-spindle-bg.jpg

Wildkart makes 3 17MM hubs with bearings pressed in. Short medium and long. The medium is what’s on most sprint karts it will give you the width your looking for. Just get a pair of them around $50 and some metric pattern wheels and your all set.

Brian