I had my kart on the scales this winter when I was installing a new seat, a friend and I played with moving the engine. Basically what we found is that the engine placement made very little difference. If we moved the engine roughly 4 inches it would only change the F:R bias 0.2% cross would not change at all. I was getting more weight transfer by putting my hands on the steering wheel vs my lap.
This was with my TopKart with a TM k9b. Engines don’t weight that much for a kart, yeah it is 32 lbs but when it is right next to the driver that weighs 150-200lbs. The driver is going to have a much greater influence on the kart.
Not to mention how the engine is mounted to the chassis vs a seat.