G'day Craig,
I'd say that what you need to do, is think about making up a new cable from any unique bits [the adjusters and end nipples] of your old cable, and new components.
The new parts are available from suppliers who cater to the vintage motorcycle fraternity, who make up cables as a matter of routine. Not that they have much alternative...
To start with, the old cable has to come off, so that it can be measured up. If there's anything tricky about its routing, take pics!
One local online source with a good range of throttle and clutch cable parts is
BMH Trading. No first-hand experience with them, but they have the right bits at reasonable prices.
A couple of files with good cable makeup tips are attached below.
Before I get into serious detail, are you comfortable with doing a bit of 'soft solder' work, with Tin/Lead solder?
It's not in any way rocket surgery, but does require some manual dexterity, and a bit more gear than a small leccy soldering iron.