Kroil is good stuff, but if you can find it, try Zep Twister. Have used all sorts of stuff on truck, tractors and motorhomes, that Twister does a hellava job.
With the side cover plate off, a coupla' bumps with a bar or a polished screwdriver would probably get the valve freed up.
If I had the (today's) price of a good rebuild at stake, I'd drain it, pour it FULL of diesel, and check back in a month.
The diesel flush can be dumped into an older cat (or my 5.9 Cummins)
Talk about waking an engine up,......
Regardless, that engine should get TWO lube oil changes, if it's that musty / rusty inside,.....get the damned cumbustion ACIDS gone.
Luck
GTC
