What aboot the genooine parts?
There's two 5th gear cogs - mainshaft and countershaft... and I have no idea which "cog" is needed to change the gearing... possibly both?
According to ePer, the 1.9 diesel box uses a 31T cog and a 39T cog which is 0.795 (not 0.763 as above).
The bigger benzina engines use either:
1.6/1.8 - 33/34T (for 0.971) or
2.0 - 35/37T (for 0.946)
Interestingly, the 1.4 uses 34/37 (0.919).
So..... "the easy option" only looks like the option to stick the 34T cog from the 1.4 into the 2.0ltr, to replace the 35T cog. That makes your 2.0ltr a 34/37 (0.919).
For the other engines, I dunno if just adding a smaller cog would work. Doesn't it have to mesh with the cog on the other shaft somehow?
Theoretically, if you added the 34T cog to a 1.6/8 gearbox, you are making the gearing 34/34... (1.0) so actually shortening it.
If a 1.9 31T fits in there, then you would have a useful 31/34 (0.911) or 31/37 (0.837)respectively...which sounds good... but only if it meshes with the existing cog on the countershaft.
The mainshaft gear cog is aboot £40 on the out-of-date ePer... but if they still make it and it will fit.... we can look at a groupie..