Like TinTin mentioned, normally Venhill has a wide variety of ready made cables, however ready made cables for a 60 year old bike will be difficult to find. Therefore you should make your own clutchcable from a universal cable.
Hope you understand my instructions. If not, I am sure you'll find an instruction on YouTube.
Use a caliper to measure the diameter of the inner cable. Get a universal cable which is longer than the original one.
I would replace the outer cable as well and get one which is lined with teflon (or similar). That way the friction inside the cable is minimal, which is better for your wrist
Normally one end of the inner cable is already fixed with a small "drum". Find out the size of the (permanent) drum on the other side and get one of those as well, and also one which can be
fixed on the inner cable with a screw (temporary drum).
This is a shop (in Dutch), which has all possible inner/outer cables and drums
https://www.cable-shop.nl/
(Normally at oldtimer markets you'll find suppliers like this).
To fix the cable properly:
1) new outer cable should be app. same lenght as the original,
2) put the small drum in the clutchlever on the handlebars,
3) route the cable to the other side, the clutch lever on the engine. Avoid sharp bends,
4) pull out the inner cable on the clutch side and put the temporary drum (with a screw) on the inner cable,
5) soon as you have found out the correct lenght of the inner cable, replace the temporary drum with the permanent drum and cut the inner cable about 0,5cm past the inner drum,
6) spread out the individual tiny cables (which make the cable) and solder them into the permanent drum,
7) once cooled off, cut off the rest of the wires.
You're done.
BTW: a teflon lined outer cable should never be lubricated.