Stevo - I have the Bren 2 Ms 7.62x39mm. Most packages being sold now come with the STANAG mag well adapter. As Brian46 pointed out, you insert the sleeve and do a quick 3 minute change to the magazine release button by removing a washer.
I document it here in this thread at Reddit:
Everything needed to convert the magwell came with the box that I purchased from G4C.
I have tested CPD Duramag 7.62x39mm 10 round mags, CPD Duramag 7.62x39 28 pinned to 5 mags, and DH Tactical 30 pinned to 5 mags.
All cycle well, except that it is possible to overinsert the DH Tactical mags. Of the three types, I enjoy the CPD Duramag 5/28s
See this thread for photos and descriptions of my experiences with the various mags so far:
If you have any questions about the Bren 2Ms 7.62x39mm (mine is an 11" barrel) or the magazine options please feel free to ask.
As others have mentioned, the proprietary mags are fine, they are sturdy and well designed, but the only way you get 10 rounds with a Bren 2 x39 in Canada right now is to convert the magwell. If you don't urgently want/need 10 round capacity, it's totally fine to stick with the proprietary set-up.