Ditto on the PB Blaster or similar followed by ATF or Marvel Mystery Oil. Once it soaks and you get a bar on it, it may take some back and forth rocking + patience.

Its also pretty easy to pull the heads on these... and NAPA can likely still get a head gasket or a specialist vendor will have one for sure. Maybe pull the head if the first approach does not work.

I run a flattie in a Power Wagon. It is mostly retired but does work a little each year. Two trips with 2 1/2 yards of mulch each were its summer work this year. Handled it like a champ.