It is possible to buy a new Vincent engine or have one made from newly made spare parts. These engines also can be bought new: BSA Gold Star and Matchless G80 from a company in the Netherlands, various JAP (John A. Prestwich) engines from a Cameron in the UK, and Norton Commando twins from England. Single cylinder engine road racing is popular in the UK and these overhead cam engines are available new: Norton Manx, Matchless G50, and the AJS7R, I think.
The new Norton company makes, or says they do, or did, complete Manx race bikes. New gearboxes are also available for most of these and almost all can be shoehorned into a Norton featherbed frame. New parts are available from all sorts of sources for these chassis. A person with money and patience can build a new old bike.
This is something I keep track of, rather than all the other stuff I should pay attention to.