Thanks for the replies guys. I have collected the parts for the wheel-driven speedo drive and will toy around with them to see if they can be made to work.
I have also found a semi-classic looking electronic speedo which may work. I will post updates when I have this resolved!
There's a really nice tach with built in speedo available from motogadget, but it isn't cheap! I'll be going with a simpler unit for this bike.