You would be surprised at the amount of parts availability for old British bikes, there is still a very large fan base for them. Just Google vintage Triumph motorcycle parts. I can’t speak to current prices, but back when I restored my old Triumph Bonneville it was surprisingly affordable to buy new parts. And unlike the carburetors on our old XS650’s, you can still buy brand new Amal carburetors.
Here is just a short sample of parts suppliers,
https://www.classicbritishspares.com/collections/vintage-triumph-parts
https://www.thebonnevilleshop.com/
https://vintagetriumphsupply.com/
https://www.donelsoncycles.com/vintage-parts-books
http://www.klempfsbritishparts.com/