I could only comment based on the video which, not pickin' on ya, was poor quality.
In the vid it looked just like a tensioner should. Couldn't pick up on any wear because of the blur, and obviously I was unaware that the spring was rusted up or that stuff got stuck inside.
I just posted to let you know about the wind-up system in case you didn't know, as many folks don't know how they work and after removing them they try to refit them fully extended and wonder why their engine dies prematurely.
The RT125D is just a carbed single pot version of the 250 V-twin so many GT/GV 250 engine parts should be fully interchangable.
Depending on the hole size that the tensioner fits into, in Oz, a 28mm one is only AUD$39.
And AUD$3 for the gasket.