Thanks for all the input here. Encouraged by it, I took a 4X8 sheet of plywood I had hanging around the house, cut it in 1/2 so I ended up with 4X4 and bolted decking on to trailer. Took it ot OVRP this weekend and used it to bring Kart home. Kart fit perfectly, with all bodywork on and wheels at race width.
As suggested I put it on backwards. The front wheels cleared the caster legs by an easy 2.5″ each side. The bodywork cleared the fenders by over an inch and the rear wheels only hung over by 1.5″ each side. Not enough to cause me concern.
When I re-do the decking, I will get 2 sheets of plywood and overlap the frame in front by an inch or two each side. I may go a little wider in the rear as there is no caster clearance issue.
Debating if I add a second level to raise kart so I can store stand and tool box under the kart. May not be worth the effort.
Going to replace standard Harbor Freight Lights with the LED lights. Going to put rubber isolators between fender mount and frame as I doubt fenders can take the vibration long term. Going to eventially add bearing buddys and a bunch of D rings/tie down points.