Show us a picture of the back of your tach and we can be more help.
There's possibly a wire, usually white, from ignition switch to tach, which goes "through" the tach and on to the coil. It acts both as a sensor for the tach and to energise the coil when you switch on. There's a black wire for ground.
There are also wires for the light in the tach.
But that's only one type of tach - tell us what kind it is if you don't have a picture. If it's original it will be written on the dial, at the bottom.
You may be able to get a cable from Speedy Cables in the UK, and a Swirl Pot from Axminster Panels (Andy Wiltshire) also in UK.
'67 Elan S3
'72 Europa TC