I highly doubt it, as a 20 Ga. is generally built on a smaller frame, and all the parts are sized down. If they were identical, Numrich would sell them for both models/gauges.
What I would do, is take the good one to a machinist to measure, and let them make one out of 1040-1045, etc, as it will need to be hardened and tempered.
Another place to look are the gun auctions and on eBay, as many parts show up on both. I looked at eBay though, and nothing but a safety and a set of hammers how up.
Last, you might call Popperts or Jack First. Jack can make the part.
Jack First:
http://www.jackfirstgun.com/
Popperts:
http://www.poppertsgunparts.com/
