I have heard of fake and replica TI Supers being created, but can't point to any firsthand. I don't see the problem if it's above-board and documented that it's a replica. To create a fake, you'd have to get into changing the VIN on the body.
Surely it comes down to intent and honesty on the part of the builder/seller and homework on the part of any potential purchaser? I'm building a TI Super replica for historic racing purposes in Australia. My own intent is to be able to run the mods that gave the TI Super better performance than its predecessor (the Giulia TI) and its successor (the Giulia Super), and still meet Australian historic racing car regulations. And as a 1964 car, it will run against different competition to the 1965-and-later Giulia Supers, which are in a different category.
There's no way it would ever be mistaken for a genuine car - it's right-hand drive, and the VIN and engine numbers are 'pure' TI. Nor will I ever pass it off as anything but a replica. However, Auto Italia magazine some years ago carried a story of an English 'TI Super' that supposedly had been rebodied quite recently with an RHD shell, but using all the bits from a genuine TI Super. This is where it starts to get murky.
One challenge when buying or building a car like this is to identify what factory, dealer and tuner (eg Autodelta) mods actually found their way onto the (supposed) 501 TI Supers built.
Alfa's parts manuals give some direction, but controversy still occurs. For racing purposes, you can't go past the original FIA homologation documentation, which includes specifications and photos of key items. Interestingly (amongst other things) this shows that drum brakes were homologated, but makes no reference to the fitment of the 'sliding block' rear axle locator. Yet the Alec Mildren Racing TI Super that won the Sandown 6-hour race in Australia in 1965 and 1966 was fitted with such an item, at some stage in its life. It's unlikely that this was fitted later than 1966, as the car's main competition role was over by then, having been squeezed out by the GTAs. Possibly it was provided by the Alfa factory, as Mildren's had a close association with the factory, and they were also running a GTA at that time.
'Factory updates' such as this probably occurred quite widely, without ever being officially documented. However it happened, it's now part of a genuine TI Super, but purists might debate its validity.
At least it'll create fodder for the BB.