Kohler Command Twin CV740S Backfires

I have a Cub Cadet Rider with a Kohler Command Twin CVS740S that was brought in for repair.

Customer states that another shop put a new Head Gasket on it and it hasn't run good since. Not sure was diagnosis they gave him for Head Gasket replacement.

Anyways, the engine starts and runs for a minute and then surges and backfires thru the exhaust (muffler) really loud.
I rebuilt carb, put in new Accelerator pump kit, new spark plugs, new Fuel Filter and Fuel lines and still it backfires.
I've checked flywheel key, which was not sheared, but replaced anyways.
Checked timing with timing light and timing does advance on both cylinders a little bit, so I'm thinking the Smart Spark Module must be okay.
Did a leakdown test on both cylinders and really couldn't see/hear any leaking from heads.

I do see that one cylinder does slightly foul the plug with what I'm guessing is oil, it's not wet but it is blackened and the other plug is really fairly clean.

Does this sound like Head Gaskets again?

Anyone got any ideas of what's going on.

Thanks for any help!

Paul