The simplest and easiest method of turboing the X1/9 is to fit the MkI Uno Turbo lump which just happens to be the same block (mounting point wise at least) as the 1500 engine usedin the X1/9.
If you want to go giant killing I strongly recommend you do not bother trying to convert your 1300 engine. The thermal load is seriously big and without the little modifications inthe turbo engine you will cook everything before you get past the 100bhp mark. The compression ratio you need will depend on what boost you want to run and what cam profile you use - anything more thanan 8:1 ratio is going to be problematic.
The MkI UT engine will happily tune up to about 150bhp on stock components (including the diddy little IHI turbocharger). At 150bhp in an X1/9 with somesuspension upgrades and a bit of a diet you should find that most of those 2l and 3l cars are no longer a problem - unless we are talking about riced up jap street racers.
If you want to take on thereally big boys then you need to build a slightly more exotic engine (Punto GT block, MkI UT head, GT crank, pistons and rods, etc.). On a Garett T25 you should be able to reach 230bhp and with moremid-range torque than those FWD ricers can cope with. If you got the diet correct for your car that should be getting on for 300bhp/tonne in a beautifully balanced chassis. You'll need somereinforcement in places but nothing serious (a rollcage would be a good idea though).
For more than 230bhp you need to start looking at a T28 and this is where it all starts to go wrong - The T28 simply won'tfit in the X1/9 engine bay. You would have to sacrifice some (or all) of your rear boot.
You're biggest problem is the water radiator (needs to be replaced with an aluminium double core), the oilcooler (use a Delta HF Turbo cooler) and the intercooler (again the Delta HF Turbo is your friend). Finding the items can be tricky but locating them in the engine bay is a nightmare. I've seen them...