Good luck with Your tuning project!! Better to start with something reliable and easy - time and money goes fast when You try to modify too much.
You are worried about the cost of a Abarth a112 engine - Here is a solution:
The 903 or 982 cc engine from Uno, a112, 127 ++ is basically the same engine as the Abarth engine, and You will find them everywhere for almost nothing, and parts are plentiful and cheap.
1) Valves are the same size from what I have measured
. Some people believe otherwise.....
2) Head design and ports are equal from what I have seen
. Some people think otherwise.....
3) Only difference between Abarth and normal is a design difference around the intake valves in the combustion chamber - this is easily fixed by hand with some sandpaper and a drill - I will try to take some pictures - if interested - to show the differences.
1) The 903 engine have heavier piston rods compared to the very special Abarth version, but can handle quite some revs. The main thing is to change the nuts and bolts for better items, as this is a weak point.
2) The Abarth has a displacement of 1049 cc so a bit bigger, and a bit higher compression
- Equal except maybe for the pushrod length
Anyway - for a real budget tuning of this engine You need the following - (sorry Guy Croft...You might disagree on a few details...)
Find a 903 engine with not too many miles. They can run for a very long time without wearing too much!!
Take off the head.
Hammer out the valve guides and smooth the ports
Flow the valves ina a drill and a dremel tool
Find a way to smooth around the valveseat, and open out the valve seats to match the valves
Install drill and dremel butted bronce guides (do not cost too much in the states but need modified shape)
Mill the head approx 1 mm for a raised compression
Get a 32 DMTR (or similar from a small Fiat sohc) and buy a fitting manifold
Get a a112 abarth camshaft used or new - quite cheap
Get double valve springs like in Abarth - preferrably new as I have seen a few broken ones
Rebuild the engine with new rings, bearings and gaskets
Install and try to adjust throttle linkage/carb, and different water outlet in head needs to be dealt with if I remember correctly
Install or build an exhaust
When ready You will have learnt a lot about engines, and the engine itself will sound terrific, and beg You to drive it hard, These small cars are extremely fun to drive - never tried it in a 600 but in a a112.... lots of very pleasant memories, one includes successfully chasing a BMW 730 on a very twisty, bumpy small road. I had a good laugh to say the least!!
A standard 903 with 32 mm DMTR lightly flowed with standard camshaft and compression produces around 62 hp rumour says... that makes for a quick car! But with another cam and compression, I am pretty sure You will get close to a 70 hp ~ ABARTH and pull past 7000rpm (which should be a definitive redline or so they say....he he)
With this power You will easily beat most of the "smaller modern cars" which weigh 1000 kg++, and have a lot more fun while driving it!! It will be reliable if not overrevved too much, and use approx 5 liters of petrol on 100 km. Cheap fun!!
My definitive advice: Please do this first - do the Turbo version later....... if not I am afraid You will never see Your car on the road - seriously!!
Some pics of what can be done easily at home with tools like a small dremel and drill!! I cannot find the pictures of the rods or chambers - might find them later:
Intake valves original/modified in a drill and a dremel some Years ago in a very cold garage.
Intake port before
Intake port later
Exhaust port before
Exhaust port later
Combustion chambers and intake ports before - see the casting marks in the ports
Ports and chamber after light touchup
Deshrouding around intake valve
Good luck and Best regards