4.2 is old school, not quite the horsepower of the 4.0. Probably the only real difference.
The AX5, well I wouldnt suggest running it, but like anything you can get away with a lot if you are really careful. But then, I thought that of the Peugeot in my jeep when I rebuilt the reverse set. Lasted a month.
Personally, I would either rebuild what you have and run it, or I would sell it and find an I6 and buy that, or shoot for the V6 and upgrade the drivetrain while I was at it.