The UV + H2O2 method is a big thing in the retro computing scene known as
"retr0bright". Lots of research videos on YT. Could be useful for restoring some 80s plastic boxes (effects, drum machines...). I got some 11.9% H2O2 mainly for the WHO hand sanitizer recipe when the pandemic started, but ordered well enough to have fun with retr0brighting while I was at it.
In my case, the keys are weighted with glued-in weights under the key tips, it was likely an ex-Waldorf KB37 unit, so any random other TP9 key would not have helped much, unless I messed with swapping the weight. I would not have hesitated getting a spare, but couldn't find any exactly fitting one. I made the posting mostly because of the surprise find of this mini-metal-tape material, after running around house and workshop for a while, trying to identify anything suitable for the job. It resembled some brackets on a kitchen appliance I saw which had such patterns specifically in order to to be epoxied to a glass front.
(If someone has a leftover TP/9S-37 with aftertouch and is willing to sell, let me know, I would use this good one instead and keep the repaired one as backup. As newgroove.it has run out, I already got a Novation Remote SL37 that I could butcher in dire emergencies, but I don't like the thought of doing so to a perfectly good instrument).