Your 80-year old wooden house probably shouldn't be settling all of a sudden. Something changed. Very possibly that reno in 2012. So take a look and make sure there isn't a bigger problem. If something degraded and isn't providing sufficient support, or more likely someone did something stupid, you want to address it.
As for the repairs - do you need that window to be an egress point? Your local building code will tell you. If not, then glass block is a good option.
Who would have skills to check the cause of sagging? Structural engineer?
Depends what it is. If a main beam is rotted or something, it's not hard to tell as long as you can see it. An engineer, or contractor who is highly competent if you can find one (and I mean This Old House level competent, they're rare so just get an engineer).
But you can do some investigation yourself for really obvious stuff. Get in the basement/crawl space and poke around. You're looking for anything that's moved, sunk, twisted, etc. Wood that's too soft. Look up at the underside of the floor - are the joists in bad shape? Look around plumbing - sometimes stuff gets cut to make room for plumbing that shouldn't be. Is the foundation bowing? Evidence of water damage? Also look at the rest of the house. If there's a problem with the foundation, you will see things further up. Are there cracks or bowing of walls? Roof line sagging? Etc. Basically, walk around the whole house looking closely for anything that doesn't look right. Then get the expert in to look around.
Sometimes the house just wasn't built quite right. There should be a header of some sort over the window. If it's missing or damaged, that will cause problems for the window locally.
And since you want to look out, then glass block isn't a good choice. You'll need to solve the underlying problem, then you can probably replace the window with something similar to whatever's there now.