the 75 never sold in huge numbers in the UK really, people generally seemed to go really basic (55 or 60 s or sx) or they went for more top of the range sporting or gt. The 75 despite now being considered probably the best 8v FIRE they ever made ended up in a weird middle ground limbo. Add to which they have always been sort after for other cars like centos and stuff and on top of that there is so few mk1 puntos left at all then they are getting pretty hard to find. In europe it was a more common engine, and also not only fitted to puntos like here.
They do exist, just a few months back i bought one for my classic panda for £150 including all the loom, ecu manifolds, TB... everything basically. Just keep your eyes peeled, one will turn up. Bear in mind when looking around that they are often not advertised as 75s and you can often just not tell from photos if it is one unless you can see a 75 wing badge or there is an engine pic. You pretty much have to contact people if you see one being broken to see if its just a 60 or is actually a 75.
That said, what seicento have you got? spi or mpi? sporting?