Too tall cast iron toilet flange in cement
There is almost no way in the world that your piece of cast iron here was designated for a toilet.
Edit based on additional pictures:   That is almost surely a washer drain.   Good news is it is probably vented.   Bad news is it isn't big enough for after truckstop buffet.   So everything below is still true but you may only need to bust up until you get to the wye that has the vent line.   Also if you are doing sink or shower you need to think about tying these in.
So you cannot just cut it off and add a coupling.
Step 1
Have someone come out and put a camera down there and see where it goes.   I am guessing your main stack location changed or you had multiple stacks.   This one is no longer in service... but are the pipes still connected and in good condition below that concrete?   You don't know and doing a ton of finishing work and then figuring out that this pipe is a bust... Well you would be out thousands plus tons of work time.

Bare minimum alternative:  Flush your whole house.   Means put a
crapload of water from everywhere and flush out the sewer lines.
Find the cleanout on the street.   Now... and not kidding... dump some
human crap and toilet paper down this line - not a kiddie dump, a
truckstop buffet dump.   Then dump about 10 gallons of water (or run a
hose).   You should be able to visually see the crap and toilet paper
running through the cleanout.  If you don't or it takes a while... you
need to go back and have a plumber throw a camera down there.

With this &quot;stack&quot; being open it could be clogged with debris, it could be fully functional, it could have roots in it, who knows.   This is anywhere from ready-to-go, to needs a cleanout, to can't ever be used.
Step 2
Let's assume you prove this line works.   You still have to vent it.   You do not want to deal with a toilet venting problem after finishing parts of a basement.   So really you need to design how this will vent.   And this is why you can't just cut it out and add a flange.  So you need to design how you will vent this thing.
Step3
You will have to bust up a little bit of concrete.   Based on the picture below (this is how I see your toilet functioning) you need to:

bust up some concrete.
go over 2-3 feet so you can install in order
coupling
45 wye
straight PVC
90 degree
straight
flange

This is the bare minimum and will take ~1 hour to do once you have room to install coupling.

Step 4
The very hardest step is figuring out where your vent is going to go.   It is its own question and depends on where the plumbing is in this house and your other options.  A toilet must be vented, so take that 2&quot; piece route it to something that can act as a vent.   If you are adding more than a toilet here, you can plan your venting out one location.
Step 5
The slope and height of the pvc needs to be reinforced (I am guess ground under) so you need to fill with pea gravel, drainage rock, then concrete.

Don't let these steps scare you.   Everything is very DIY friendly.   Do be scared about adding a toilet and finishing this space if you miss one of these steps.