They don't screw out if its an original coil. But you could cut it right back & add one of these then a new lead from there on to the plug cap.
www.classiccarleds.co.uk/products/in-lin...essor-5-k-ohm-carbon
There is that type of coil in early yamahas--single wire magneto coil with a screw in lead but not that common. JT2 below had one.
www.partzilla.com/catalog/yamaha/motorcycle/1972/jt2/electrical

On my '71 CT1 the HT lead screws into the coil, and into the spark plug cap. While troubleshooting yesterday I wanted to replace the original 50 year old lead, and after trying several places (including a motorcycle shop) found that O'Reilly Auto Parts sells 7 mm HT lead by the foot, and 1 foot of it is perfect. Only cost $1.09, but I probably spent $20 in gas driving around.

Solid copper core wire? 

Plug cap resistor sometimes goes bad and many people are running "R" (resistor) spark plugs with the original resistor plug cap since original non-resistor plugs are now obsolete.  Running two resistors measurably reduces available spark energy. 

The thing about spark is all you need is "enough"...  so reduced spark energy won't be noticed as an issue...  until conditions require a little more energy than you have available and then you have a misfire. 

MarkT, the old original spark plug cap is a resistor cap? I actually have an original spark plug in one hole, but when I ordered a new spark plug, even though I didn't order a resistor plug it came as an R plug.

By the way, I cut open a tiny piece of the lead wire I purchased at O'Reilly's, and it's braided tinned copper, just like the original.

Get a decent mirror & see if you can get a look at your coil incase i'm wrong on coil type.

Yes Tom, 5k ohm resistor cap is original