Hi Martin.
Treated myself this year to a Rev counter for my 1960 Super Rocket, always wanted one, found it strange on a sports model not to be fitted as standard, only as option, but standard for export. My headlamp nacelle was removed many years ago and replaced with separate headlamp. So I have twin clock arrangement, positioned side by side. Using nacelle with separate Rev Counter might look a little odd, only my opinion. Your drive should be via the oil pump, you will find blank plate on lower part of inner timing cover, this is replaced by drive adaptor and cable.
Rev Counter Part No - RC1303/03, 3 to 1, anti clockwise.
Hard to find now and even rough examples go for silly money, any chronometric speedo or rev counter could be converted by specialists, I used Ashley Pople @ Speedo Repairs.