Ive never had any problems with my 3900, and I drive it hard. And I mean like, I'd be kicked off the site for causing so much stress on the g6, lol. I dont see why they have all these problems. I guess if you redline it for a min straight, you could blow a head gasket. And Idk why there is no parts for it. Only problem ive had is my intake was sucking in hot engine air, and caused it to bog a little. But I fixed that with a little work. But about the internals, I'd say it couldnt be to hard, if you found or had parts made, to install them. Just so much bs in the way. And I think it could handle it with a little tuning. Maybe not the trans and driveline, but im sure you could find something to sit that engine on. And as far as aftermarket head gaskets, idk what to tell you.
2006 G6 GTP: 4speed auto, intake resonater delete, mild weight reduction, some custom work, lol.