To remove the metal forend flange, the rivet has to be drilled out. Start from the inside of the forearm with a drill bit that is larger diameter than the flared rivet.
Remove the flared part of the rivet, and drive the rivet out of the metal piece and wood forearm. The metal should slide forward.
I would check with either Brownells, Cole's Gunsmithing or Midwest Gunworks to make sure you can find a replacement rivet first.
I've done 390 forearms but never the older AL-2 / 301 / 302 / 303 forearms.
I know that you can still get rivets for 390 and 391 forearms, but I think the AL-2/301 rivet is a larger diameter.
There is a guy that sells older Beretta parts on Trapshooters.com
He goes by ACSS his name is Wally Riebesell