Good morning Richard and welcome.
CK guidelines/rules can be found here...
http://www.cyclekartsgb.com/what-is-a-cyclekart/what-is-a-cyclekart-and-rulesguidelines/Or simply put, Honda GX200 or clone, allowed to go to stage 1 mods, so max 9.5 - 10 HP, I don't think any of us have actually felt the need to do this, all I've done is changed to a tuned exhaust to give a boost to low revs torque, and removed the screw that keeps it's revs to 3500, I think I get all of an extra 300 RPM! Others have changed carbs.
If I could suggest go with bog standard engine, drive the wheels off it, have a blast, and change things as and when you feel you must :-)