Limitation is :

I can't look over your shoulder and see what you are talking about, online, in text..

Do you have a place to sell you just the cage? or are you cannibalizing a broken derailleur?

I have done that with an 80's Campag Olympus. [Back when they were dabbling in the MTB sector]
but they are better, with spare parts than Shimano..

I'd just get a new RD.