My factory parts list shows an ejector spring on 20 gauge guns.
The problem with drilling out the rivet isn't so much how difficult it is to re-rivet the new spring onto the ejector. It's that you have to re-blue the receiver when you're done to blend the head of the replacement rivet.
Brownells stocks a rivet cutter that sometimes allows you to take off just enough of the rivet to re-peen the rivet over a new spring, thus preventing a re-blue on a new rivet.
http://www.brownells.com/.aspx/pid=3...G-RIVET-CUTTER