Without going into all the technical info/jargon, If LPG was the answer everyone before you would have been using it. I am all for thinking outside the square and doing things differently however, some things are best left alone in my opinion. Having said all that tho if your that super keen, give it a go and let us all know your outcome. 

Unless you are attempting uber horsepower with a pinto motor you are looking into this too much.  The std pinto motor has a lot of mechanical design limitations, which would cost a lot to fully rectify.
 
If you want a turbo charged street engine look at the one joeyjonsey has done.  You can easily turbocharge vitually any motor, especially for minor power increases of 50-100% with the std motor.
 
The important 3 aspects of any motor for performance are cam, compression, and airflow.  Everything else is just icing, fancy rods and forged pistons increase durability.
 
The pinto cam is pathetically small with very little lift, it is cut on a wide lobe centres and generally installed retarded.   
 
You don't need to really lower your static compression for mild power increases(especially with a LC bottom end),  8.5:1 is generally considered a good compression.  Even then you can fine tune compression with cam lobe centres.  Also if you engine cranks at less than 160psi you can throw 8psi at it with no problems.
 
Finally the turbocharger has the biggest effect on driveablity, ideally this should be sized correctly especially on a small 4 cylinder.  Whats the point of have a turbo that produces max efficiency at 30psi if you are only going to 10psi?  In the same token why have a power peak of 7500rpm when your car spends 90% of its time between 2-4500rpm? 
 
I am in the slow process now of turbocharging a mates mk2.  I am hoping to do it all for less than $3000.  That includes:
 
Fully ported head
Custom cam
Std bottom end
T3 turbo
Fabricated exhaust & manifold
Camira Ecu & wiring loom 
Fuel pump & injectors
Modified carby manifold
Ebay intercooler
 
$1500 of that is going to be the head and cam.  We are doing about 95% of the work ourselves.  Hoping for 220fwhp at about 10psi.