If what you have is indeed a factory built abarth you have a very valuable car. If it has 600D badges on it anywhere then chances are it isn't.
Abarth built cars, modified production cars and sold hop up pieces and kits. I would say at this point 90% of all vintage fiats in the USA have abarth badges on them somewhere.
If you have one of these in the engine bay chances are better
But even these have been avaliable through the aftermarket for 30 years.
Now as to your missing block a D would be 767cc 29hp. Luckily that is the most common proper 600 block.
If you are looking to do a proper period correct restore/hot rod look here
http://users.ntplx.net/~kinukoyc/Pages/tech_pgs/engine_how_to.html
If you want extra power just find a Fiat 850 motor with 43-50hp.
For parts look to
scuderiatopolino.com <--Mega awesome race oriented stuff...pricey!
fiatplus.com <---Everything you will ever need served up with a healthy dose of patronization. Not great pricing but a bad attitude and being the worlds foremost authority in rear engine fiats should make up for that right? Used parts descriptions are sometimes way off.
mrfiat.com <-- 100% awesomeness who will go out of their way to get you parts they don't stock.
Lately I turn to global.ebay.com and get the goods from italy.
Your 600 has almost nothing in common with the 500. Two totally different cars and your watercooled 4cyl isn't related to the aircooled 2 cylinder at all.
mirafiori.com is the hangout for most USA based fiat owners. I wish there was a better forum and have thought about building my own but I don't think anyone will leave mira while it still lumbers on in its 1992 based software.
Oh and please post more about this car. Pictures, anything. I LOVE 600's. Still looking for another (I have had 4)
I daily drive its slightly bigger younger brother the 850 Berlina
If you would like to know how to make a reliable 120hp out of 843cc look at my readers ride.