Well, I have a 1050cc motor built on a 903 block. Has A112 crank, rods, cylinder head, oil pump & performance cam and some various special parts. The main journals on the A112 are larger dia. and I had them ground down to 850 size. 67.2 mm for pistons is about as large as you want to go on the 903 blocks and requires offset boring of the cylinders and needs to be done by a machine shop that knows what they're doing. The cylinders need to be notched at the bottom and the cam needs to modified to clear the rods. The A112 oil pump has a built-in pressure relief valve and IIRC has a higher flow rate. I'd have to go look up the info. on everything that was done. I collected all the special parts over a period of 2-3 years or so and then sent everything to Paul V. (S/T) for the special machine work and assembly. I've got over $10K into the motor not including DCD carb and exhaust...
It will put out up to 90HP.
If you're going to be running much above stock HP, you will most likely be replacing the rubber-filled axle couplings every other week especially if you have a heavy foot... A CV joint setup can be done, but is more $$ and some special shop work is needed. I bought special hub carriers from S/T and 128 CV joints and axles. The hardest part is joining 850 axles to 128 axles. And with a lot more HP you may find you'll want to upgrade the suspension and maybe the braking. Again, more $$.
Another option would be 65mm pistons with an A112 crank to get 982cc. Can't recall offhand what rods are needed for this.
Personally I wouldn't run a PBS head for regular street use unless you have deep pockets to do everything that will be needed, not to mention maintenance time and expense. A bone stock A112 1050cc motor is rated at 70HP and IMO a very good option if you reverse the rotation. No more centrifugal oil filter! Not sure what these motors are going for these days and will be hard to find in the US. A few years back I think they were something like $5K for a decent used one.
If you haven't read these sources, they have some excellent info.:
Paul V.'s old S/T info. on building a high performance motor.
http://www.fiat850.co.uk/download_store/documents/other/Scuderia_Topolino.pdf
Mahlon Craft:
http://users.ntplx.net/~kinukoyc/Pages/tech_pgs/engine_how_to.html
The Guy Moerenhout forum in Belgium will have some useful info:
http://www.abarth-gmr.be/new/
Many of the parts you'd want can be found on ebay Italy and if you spend some time looking through what's available, you'll get a sense of what kind of money you'll be looking at to build a good high performance motor.
I'm sure others will come along and make more comments.