Gday Trav and welcome...first thing to do is buy another bike while you sort this mess out...Tassie riding is too good to miss out on...
If you got this far you might as well finish the job and rebuild the trans completely - at least then you will have peace of mind. I've replaced gears with only 10k km on them and the dogs were already badly worn.
Isrub's suggestion would probably be the most cost effective but you have to find a donor bike and most likely get it across on the Spirit somehow etc ...lots of time and risk in an already worn trans.
Wow I wish I had your garage and resources. I am looking at maybe the same 4th gear is messed up on my 2011 R3R right now, started yesterday. First step to drain and drop the sump to see if can see any shards or bits of cogs etc.
I've borrowed that (and the bar that goes with it) from the dealer a couple of times. I had planned to take that schematic to a machinist along with a couple of cartons but haven't got around to it yet.
My alternative was to source a bolt that matched pitch and diameter and taper the end to fit...but haven't got around to that either...i have an 11 month old so spare time is a long forgotten luxury...
A 3rd option is order one from Triumph if you are feeling wealthy!
We will take this opportunity to summon @Mittzy and @Ishrub who might know of one on the east coast somewhere...both are resourceful fellas