classic motorcycle forum

An Ebay search is your best bet. Click on the "completed items" tag in the search list to see what's been sold. Some of that stuff, like the Bonniksen speedo, the Speed Twin engine and the Velo headlamp will probably surprise you when you see how much it could fetch. Even the price of rigid Matchie frames these days makes your eyes water...

I don't blame you in respect of a lost split-link (I do carry a few essentials such as this) but I wouldn't dive into the gearbox by the road-side unless I really had to. It's not enjoyable.
Anyway, you need the 1964-on gearbox end cover, easily identified by the later and better three-ball lift mechanism.

Nothing's "impossible" where engineering's concerned, but done properly there's every chance the cable will outlast you.
Those (from memory) pear-shaped nipples on the lower end are available, as are all the other nipples such as barrels, ball-shaped and even the little tits on the end of choke and throttle cables.
I find "www.trialsbits.co.uk" pretty good for cable stuff.

Anyway, even if the worst happened and your clutch nipple pulled off after you soldered it, you'd be no worse off than you are now with the pattern stuff failing...

Powder coating (in my experience) isn't that good; you can't use filler, nothing fits together afterwards and any trace of oil or grease leaves pin holes in the finish. Sure, it's tough, but then so's stove enamelling.
I recently had a pair of girder forks powder-coated, and even though every hole was plugged (either with studding nuts and washers, or sacrificial screws) the blasting media found it's way into the spindle area and it took a lot of shifting. The thought of some of that crap being left in the OIF oil tank tube is frightening.
I don't know what the answer is, but for your frame I wouldn't use anything "blasted" which is likely to remain and cause damage. Clearly there are options as so many T140s have been restored. Maybe ask on the Triumph Twin Yahoo group?

60? More like 70+ unless they're just plonkers acting the part....

$5000? Snap his hand off.... and go and buy something you can actually ride on 21st century roads. A utility 125 is pain rather than pleasure.

Good that you've got it cracked. I've never worried too much about modifying components in this way, after all, it's better to be riding it than obsessing over not having the correct 1962 squiggle washer. Mixing and matching 50+ year old parts can be fraught at the best of times anyway, and that's with a mass-produced bike too.