Skoda S 110 gearbox fits very well.
I have seen one fitted to a 126 but I was told by a couple of Skoda specialists that it is not the strongest and often fails with tuned Skoda engines. The Renault Gordini/Alpine turbo had 108bhp as standard and can take some more, especially if it is lower torque power delivery that is normally associated with normally aspirated engines.
The only problems that I can see is that the box has the lever the change on the side rather than the tail and it is quite a long box. The first problem can be got around with a well thought out linkage or the tail section form a Renault 8 or Daphine which can be modified to fit, the second could be more of a problem as cutting the body to make room for the gearbox may mean that the car needs to go through the new BIVA test as this would be classified as modifying the monocoque
