The issue was never fully fixed, my 2014 did it. Bought at 0 miles, trans failure in less than 5000, but 2nd was slipping out long before that.
You will need both 2nd gears (in/out), detent spring, clutch nut, front cover gasket, all circlips on both input/output shaft, the anaerobic sealant, special tool for stator/flywheel removal, updated clutch cover plate, updated lifter, updated lifter fork, and the washers etc in that area, reg/green/blue loctite, stator cover gasket, head gasket, new or ARP head bolts, and Triumph say case bolts should be replaced (I used ARP because if you ever have to split it again, ARP just saved you money, not a kit, order by size).
The parts bill will likely be around $1500.
DO NOT split or try to split the cases before you get the stator off and remove the 3 bolts. If you do, you'll break the mounting tabs there.
That should get you a reliable trans again, but I may have forgotten some pieces. The update kit changed a lot of little stuff, some more necessary than others. The easiest way to get all the updates, is compare part numbers between classic and roadster. Buy all the roadster trans/clutch stuff that has differing part numbers/diagrams.
Or....buy a crashed Roadster or find one being parted out and buy the stuff from it

Most of the update parts are low/no wear items.
eBay yields no good candidates currently, boo.