Hi

I have a speedy with orig telemetry glass/bezel which is very scratched. I’ve managed to source a replacement and ahead of a service next year, I’d like to have it fitted

 

It's not a big deal for person with the skills and equipment. My watchmaker did on my 125 several years ago without any problems.

If your question is if it's a DIY easy without the proper tools and training, them the answer is definitely "do not do it by yourself".

 

It's complicated in a Mark II. I can imagine that it's the same in a 125 because the glass can break.