I did this on my old Alfa 145.. I fitted the two fifth gear cogs from the 1.9 diesel which was sold in Europe but not in the UK. It left all the ratios unchanged but top gear had less revs on the clock for any given speed so it was a lot less "busy".
You need to find out the gear ratios that each car with a similar gearbox has and then identify the one that will give you a decent change in final drive ratio... it's a lot of hassle just to save 100rpm at 60mph for example.
Failing that, if there's people that can make you a set of cogs to order, then that's maybe a goer but even an off-the-shelf cog is going to cost £100 so £200 or more for a matching set ... you need to really want it.
On my 145 I estimated I would save enough fuel for the mod' to pay for itself after two years... but a Cinq' doesn't exactly knock back the pints, so this mod' would only pay for itself after 3-4 years, I'd guess.
Best idea "on the cheap" is to just fit wheels with a bigger rolling radius, for example some 14" from a Mk1-2 Punto. The drawback is that the "higher gearing" will affect all the gears so if you think your 900 is a bit sluggish to accelerate now, it will be even slower with the larger wheels/tyres. But the advantage is that when you're not touring you can fit the standard wheels, whereas swapping gearboxes is a slightly more than 20 minutes job.
Ralf S.