If you can't afford a 1,000cc Vincent, how about something smaller from the range?
http://ridevintage.wordpress.com/page-22-1954-vincent-firefly-on-raleigh-cycle-with-webb-forks/
There are a lot about, but overpriced for a friction drive I suspect because of the name. A similar and probably cheaper device was the Garelli Mosquito, and there are more of those around too.
I can never understand why devices that were a sign of poverty, a black and white and dreary world, deprivation, in a nation still finding it's feet after a major war are so "valued". They were dreadful but necessary then, now they're just dreadful.