As far as I can see from the photo, no modification has been carried out on the cylinder head in order to fit this carb. It does however, seem to be a big carb for that engine. Did it come with the engine? If it did, there is an evens chance that the jetting is correct. If it has come as a 'to be bolted on' modification, I would suggest that you find out what engine it came off in order that you can find out what jets etc. are fitted vis-a-vie what you will need for the 650 engine.
Alternative carburettors?----strewth, where do I start? This can be quite a contentious subject! Personally, and from my own experience, I think that the best alternative carb for the 650 engine is the Dellorto FZD--30/24 or 30/26 for a standard engine, 32/28 for a tuned engine---single, simple, fairly light and stay in tune. The downside is that they are now getting rare although if you are prepared to spend some time trolling through the Italian sites, they are still available. A word of warning--there is a 'Chinese' copy on the market, discernible by the lack of the word "DELLORTO" cast onto it---do NOT buy one of these under ANY circumstances--you WILL have problems! Alternative carbs include the Keihin and Mikuni bike carbs (manifolds available 'over the counter') and the Dellorto OVC 34/30D (must be the 'D' variant)--a sort of side-draught FZD, and still available NEW! If a 'twin-choke side-draught' carb floats your boat Weber, Dellorto and Solex all make versions for which inlet manifolds are on the market. The Dellorto and Solex carbs are substantially lighter than the Weber, and their venturi sizes (29 and 30mm) are more suited to the 650 engine. All the side-draught carbs require supports, again easily available. Lastly, and not to be ignored, is the standard carb that Fiat fitted, the Weber 28IMB. Yes, it is a simple carb but for a standard engine it will do its job perfectly adequately. All carbs are, to a greater or lesser degree, a compromise between performance and economy---"yer pays yer money and picks yer tune"