Have a look a
More.groupware (I use it).... should have most of everything you'll need from a groupware perspective (uses a version of Horde mail), and has a minimal ammount of bug-trackability
... as for true bug tracking... BugZilla comes to mind, but don't know how close to mForge it is. Both of these are standalone... and it would take a Mountain of work to make 'em Mambo-able