Hotter cam and bigger carbs are definitely the way to go these days. I say 'these days' because there are so many fast cars around, what's the point of trying to compete with a 3L twin-turbo Nissan... you should instead preserve the unique Italian character - or more correctly, give it the character it should have had in the first place if it weren't for emission control laws and production cost cutting!
Yes, the Uno Turbo has a great engine (I own one) but I wouldn't put it in my X1/9, it just doesn't have the right character and at the same time it's not the 'ultimate power' (105bhp standard) that you'd need to make an X1/9 compete with newer turbocharged cars. Yes, lots of people do it because it's an easy enough swap. And it's super-reliable and economical. But I think, by the time you've got hold of one from overseas and spent $$$ putting it in, you might as well have bought a supercharged MR2 (or something)...
With a ported Yugo 1116cc head, PBS camshaft, and twin Weber 40DCNF (or 36DCNF carbs), you'll be looking at about 120bhp - that's a nice increase over the 75bhp, but the figures are only half the story - the engine will also sound great and give a real 'zing' to the drive. A fuel-injected, turbocharged engine does not do this. I think that anyone that's heard the induction noise from twin Webers (the chortling sound at 7000RPM especially) and sensed the lack of a flywheel as they attempt a fast gearchange, will agree with me there (I had a 128 Coupe with a 1500 in this tune...)
Since you have the injection model, you already have the most powerful (and reliable) X1/9 ever sold in the USA, so you may also elect to leave it just as it is! But when these engines were designed in the sixties, I doubt that Aurelio Lampredi dreamed of electronic injection with airflow metering and a catalytic converter. Instead, he would have chosen the twin Webers and a 4-2-1 exhaust...
These are just my opinions (of course)...
-Alex