I`ve seen some of the 126 with the 127/903cc engine mounted to the standard gearbox, but I`m not sure if you need an adapter plate for this.
I know the 903cc engine bolts straight on to the 600 gearbox, as we`ve done this before. The 600 gearbox is also a bit stronger, so this would be a good option. I never had both gearboxes side by side, but I can`t imagine there would be too much difference.
I was playing with this idea for ages, as the 903cc engine is very sturdy, gives good MPG and should be easily and reliably good for about 50-55hp.