Hi Jim - Sorry to hear of your 'fun'
Coming from the ISL, a CAPS era ISL (But believe the VGT era had the same issue to...) - cracked or warped or blown gaskets on Exhaust Manifolds was explained to me as 'Not IF, but WHEN
!' it would happen.... (We had a cracked, as wells warped flanged, as well as blown gasket... Number 3 as I recall. Assume the gasket blew first, the heat built up caused a bit of warping, and the crack was a result of the warping.... But who knows
!). This was in our previous CC Allure.
ISX's are also capable (As you know!) of having problems. Here is an thread on Rawze's site. And note the comment about the three pieced design, and the 'flexing' or expanding/constricting of the pieces. And a comment about rusting expansion joints, limiting that ability.
The real decision point, is to replace it with OEM or After Market... Two schools of thoughts on this one... Where in many threads the comment about an engine needing to produce 650-750 HP range - before a aftermarket polished and ported Exhaust Manifold would really yield any benefits... (Though many say 'Any time an engine can breath easier, with less restriction, the more HP per gallon of fuel burn will be generator.'.)
Me? Well I like to fix things once if possible. And with labor rates, and PTIA of coming in from the top side of our bedrooms being involved - well, perhaps the aftermarket better alloys start making it an OK investment. And heck, if the engine does breath bit better on the exhaust side, certainly can't help...
On the ISL, no aftermarket Exhaust Manifolds were available, at least that I could find back then
I would not go with just a new gasket. And you'll read in this linked thread, and I've read this elsewhere too - that machine Exhaust Manifolds can be problematic...
If ours goes out on our 07 Magna's ISX600 CM870... Well, I'll do a deeper dive on aftermarket units...
Will follow your post, and learn from others input... And would appreciate your final approach to how you repaired yours.
Best of luck,
Rawze.com: Rawze's ISX Technical Discussion and more - Exhaust manifold