there are many carbs you can use.
original Abarth are:
- Solex C28 IB-2 (very hard to find!) I've got one and it is very nice to drive. It has a 25mm venturi (Weber 28 has a 23mm venturi). But I do not sell
- Solex F34PBIC on the special valve cover. Original ones cost about 600 Euros. There are offered many PBIC, but mostly not fitting to the Fiat-2-cylinder engine... And it's hard to find the fitting idles and even more hard to find the correct venturi. I have one, but do not sell. I could sell a different 34PBIC, but not fitting venturi, idles etc.
- Dell'orto FZD 32-28. Many use that carb and it is said, it runs good...
- "Il Motore" in Hamburg, Germany offers Webers made up to 30mm. I've been said it is quite good:
http://www.ilmotore.de/shop/index.php [nofollow] Just ask him.
If you want original Abarth, you should look for a Solex carb. If you do not mind, then the Dell'orto would be a good choice...