How do you replace a steam boiler site glass that cannot be seen through.

Buy the glass. Use a special inside glass cutter to cut the glass to the proper length. Replace glass with new rubber gaskets. You are done.

You can buy a sight glass from a wholesale house or heating contractor. Remove the old glass by loosening the nuts on either end of the sight glass. There are rubber gaskets under those nuts. Along the lines of a compression gasket. Get new gaskets when you get the sight glass. Put the nuts and washers on the new glass and install and tighten the nuts.
Do not overtighten

I never tried this but wouldn't a nice soaking in CLR overnight clear the old glass. Worth a try?
Just thinking out loud.

You can clean the boiler sight glass without removing it.
I googled clean boiler sight glass ammonia after I broke one.
Note - I am not a plumber, but this worked for me...

http://www.superiorspecialty.com/Main/Heating/Topog-e/maint.html