The engine parts are manufactured in Europe and they have a distribution system here in the US, but I don't know for certain if they were actually made by Fiat, or a supplier. I had to scavenge several parts off of a salvage long 445 for this project, so I know a lot of the parts are very similar (some minor differences in quality control were noticeable). I've read that this was due to the Olivers being made in Italy and the Long's being made in Romania I think, but I don't know all the history there. I believe your 445 should be similar to an Oliver 1255, and engine parts are definitely still available for the 1255, so it would be worth a call to find out what a full rebuild kit would be for your 445. As far as availability, Rick had nearly all the parts on the shelf, so I had them in 2-3 days once ordered. Danny Bowes, up in Canada, has a nice website that shows the Fiat/Long/Cockshutt/Oliver/MM/White/Allis/Hesston equivalency tables so you can see what your Long crosses to. I bought several parts for this project from him as well, and have been happy with everything except for the brokerage fee that you have to pay to import stuff from Canada to the US, and the amount of time that it takes stuff to clear customs adds a few days to normal 2-3 day delivery.
If by chance you have a "good" New Holland dealer nearby (I do not), since they are owned by Fiat, you can get Fiat parts directly from them that fit. My problem down here is that I have to take the New Holland part number to them and then they tell me they don't have it in stock (which I already know), but they can have it in 3 days if I can wait, which I do. I've found that the rear ends on New Holland model numbers 3010S, 4010S, 5010S, TT55, TT65, and TT75 tractors are nearly identical to the Fiat Oliver rear ends. So, if you have a good New Holland dealer nearby, you have a decent chance of having rear end / final drive / PTO / 3pt parts all in stock locally.
Sorry for the long response, this was probably more info than you were looking for.