Hi Piet, Welcome to the forum,
Steve Smalllcox is running nearly 100bhp through his 126 gearbox, What it boils down to is the way you use the clutch, If you feed the power in gently you should not have a problem but if you drop the clutch at 4000rpm you would strip the gears

the alternative is using the 5 speed box from a Skoda 110/120/130 but to use this gearbox you have to modify the floor under the rear seat as the gearbox is taller. I like the idea of fitting the Toyota 3 cyl engine, its small and light very similar to the Matiz, although i would imagine the worse part would be working out the wiring for the ECU, It would be good if you started your own thread in
[Your Restoration] Section so we can all see how the project is coming along
