I created this helpful info-graphic and instructions for current or future use, depending upon whether you have gone full caveman on the wheel at present, or not.
Since we're working against gravity, you should turn the kart over or completely upside down (front and rear supported on saw horses, barrels, boxes, oil drums, etc.) so the fluid will run into the rusted joint. Next, saturate the rusted joint with penetrating fluid; Liquid Wrench, PB Blaster (or the stuff Poboy swears by).
Application of penetrating fluid should be repeated after several minutes, hourly, over several hours or days if it is still stuck. Give time for the penetrating fluid to work as it flows into the threads (if it has them), crevices, nooks and crannies.
At the same time, get thee a ball peen hammer (for metalworking) and a punch whose working end is not much bigger that the object you are trying to move. As in the below illustration, use the hammer and punch to pound the steel insert of the wheel in a circular fashion at 12, 3, 6 and 9 o'clock, for example. This should, with medium to heavy force, begin driving the wheel off the shaft and if not that, will create helpful vibrations in the steel to help break the rusted parts loose. Keep going around and around at different spots on the steel insert. Do not pound the plastic of the steering wheel as it is too soft and would absorb some of the vibrations you are trying to generate. Keep applying these methods until the wheel comes off or is loose enough to pull off by hands alone.
The next time one encounters rusted parts of this nature (and who can turn down a basket-case kart or minibike?) the same methods can and should be used.
I would also suggest the use of heat from a blowtorch to help free rusted parts but that is as a last resort and would damage or destroy the wheel.
