Bryne,
Despite the flattery from Chas
I haven't actually upgraded the turbo on my Mk1... it's standard!
I think you need an IHI turbo off a Mazda Familia or perhaps the 2.8 Diahatsu Diesel turbo that I bought a year or two back. I never got around to installing it; now I've lent it to a friend to see if he gets around to making it work.
Bear in mind that a larger turbo will absolutely have a disadvantage to weigh against the increased power: more lag at low speeds. I think even Dunc would agree that there is a compromise involved with turbo engineering (and I certainly admire his turbo...) FIAT/Abarth spent a while toying with this compromise when they designed the Uno Turbo. I'm sure that the actual size of the turbo was not chosen by cost, but instead by driveability/economy/reliability issues.
What you also have to remember is that extra air requires extra fuel as well. Some people upgrade the size of the fuel injectors to increase the fuel supplied by the standard ECU, but since the ECU lacks any feedback on how much fuel is actually going in, I think this approach is pretty crude. Really, you are going to need to replace the fuelling system with something aftermarket. You could add a 5th injector, switched on by the 'safety' switch that normally activates at 0.8bar or so. But again, this seems like a crude fuelling control to me.
I've decided to keep mine standard, with the boost cranked up a bit using my simple bleed valve. I found during a hot day at the track that I was losing boost pressure, so I'm going to put more effort into cooling/water spray on the intercooler etc.
Somehow my Uno Turbo has something weird going on in that the boost gauge registers most of the time and the fuel economy is exceptionally good. I've decided that it wasn't worth changing!
It's all about cost and what you want to achieve. That's easy to say! But it's true... Driveability is important (unless you are perhaps mainly using the car on the track). Economy is nice to have. Reliability is useful sometimes, more often than you would think.
If your standard turbo is stuffed, I would be looking out for a standard one for sale after someone has 'upgraded'. Otherwise, if you are going down the bigger turbo route, be prepared to plan for an aftermarket fuel system for best results. And, how is the rest of the engine?
-Alex