the threads in the case appear to be stripped.
My thought is to have the drain hole welded closed and then retap
Nah - Helicoil or Timesert the existing hole, the original thread is 1/2"UNC. If possible, try and find an individual or company locally that does 'em, or you'll be shelling out for ten inserts and the tap ...
When selecting the helicoil/Timesert, be aware that they come in various lengths, each being a mutliple of the diameter - i.e. in this case, '1/2d' is 1/4" long, '1d' is 1/2" long, '1.5d' is 3/4" long, etc., etc. Measure the thickness of the casting and select the correct one. Whatever you do, don't do what the muppet who put my 500 engine together did - put in a longer helicoil protruding into the gearbox;
I couldn't work out why only about 200cc of gearbox oil came out ... 'til the rest spilled all over the garage floor when I took the cover off ...
level tube drain plug (T3668).
new level tube plug.
I wouldn't replace with another 'level tube drain plug' - ime, the originals have a propensity for the level tube coming out of the drain plug;
if you're lucky, the tube just rolls around the bottom of the case; if you're unlucky, the loose tube was picked up between two gears, whereupon the engine seizes immediately, and breaks the gearbox case.
Given the general standard of 'cheap bits' available today, I wouldn't trust a new one to be any better made.
Iirc, from '71, Triumph fitted an ordinary bolt and changed the gearbox inner case to incorporate a plastic dipstick.
On all my Triumphs, I've replaced each drain bolt with a socket cap (Allen) screw sized to protrude just into the case with, outside, a spring washer and plain washer compressing an 'O'-ring - Allen screw because the hex. key is shorter than a spanner so I'm less likely to strip the case thread, 'O'-ring 'cos it makes a better seal than a crappy fibre washer.
T160's have gearbox dipsticks as standard, the T100 I've changed the gearbox inner cover for one with a dipstick. I've a later gearbox cover for the T150 but, to be honest, I don't miss the level tube - I've long poured gearbox oil into a cheap enamel measuring jug (from a kitchen shop) before pouring it into a gearbox; the gearbox oil can't go anywhere unless there's a leak; if I want to know how much is in the 'box, it's hardly a chore to drain it into the aforementioned jug.