Joe, you and I should swap stories over a beer someday. As you already know from many previous posts, I raced all through the 80' and early 90's in a laydown. Still have my sit up sprint kart converted over for road racing, but it's so out of date that in maybe 5 years it would qualify for vintage racing.
I raced a poor man's enduro in the 70's as well and got to Riverside, Ontario and the old Orange County Raceway drag strip and bike road course. I've got some 8mm movies of that I should dig out.
Full bodywork WKA style. And it's not that much slower than my old laydown enduro. Only at the top end and it will come within 10 MPH of top speed at around 90 geared appropriately.
I would do a few laps, but not race anymore. I know a full race in a sprint kart, shifter or single gear, would kill me outright. I knew an older guy (however, he was a smoker) that did die on the course of a heart attack. He was in his late 40's as I was at the time.
Karts are about the same as motocross as far as overall exertion goes. Maybe more in a couple of aspects. Formula racers in IRL and even F1 compare karts to their cars for the amount of effort they take with one exception, the G's formulas produce.
With no downforce, a kart is good for about 2 G's in some corners and braking. And, with no belts, you have to hold yourself still in the seat with upper body strength because if you flop around in there, the kart starts to handle like crap. Not many drivers use any Body English while driving unlike all the movement on top of a bike.