I have recently put oil in and got the engine running, but I have now spotted a small leak from the mechanical speedo connection.

I replaced the oil seal when cleaning up the T9.

As I am going to use an electronic sensor do I need to blank off the little gear wheel.

Suggestions for fixing the leak please

mine is just blanked off, as like you I have an electronic sensor. Neat aluminium plug,...

I just span a piece of ally bar on the lathe and made it a nice fit
(Added a little bit of silicon to finish)