M12 and 42 barrels attach to the receiver by way of a interrupted thread barrel extension. Fitting a barrel to the extension will require a smith. Even if you locate a "complete" 20 ga M12 barrel (with extension andmagazine tube) the M12/42 takedown system is very different than that used by Remington and Mossberg. It demands closer tolerances. Therefore, the short answer to your question is: no.
It is possible that you would stumble upon a barrel/extension that will just happen to work. But this is unlikely. The good news is that if you find a lost puppy 20 ga barrel/extension it can be fitted to your receiver. I can recommend Simmons for such work.
Sam
