First of all the retractor reel is spring loaded, so you will not only be trying to remove the webbing from that reel, you will be fighting the recoil mechanism that retrieves the belt when you punch the release button. Why? Because to get to the end of the web that is mounted/attached to the reel, you have to extend the web belt all the way out which has the effect of winding up the recoil spring to about tight. Unless you can figure out a way to lock or freeze the reel from moving against the pull of the spring, there is a very good chance that once you remove the belt, the reel is going to wind up again, that being its normal behaviour pattern, and that will mean that you will have to figure out a way to wind the reel back out again to bring it back to where it has to be to install the web and then have it retract correctly.
For myself, my vote goes to the salvage yard crawl and scoring a decent complete replacement. Both less frustration and less likely to result in getting a finger or two nipped or ripped during trying to disassemble the retractor.
Nick