While the other head is off, I strongly suggest you try that wet leak down test, fill all cyls to brim as possible, especially #5, then see which cyls are sealed. It's an inexpensive valid test.
You may be very surprised. ( Diesel is kinda on thick side, stay with solvent/paint thinner viscosity stuff)
The test is a comparison, to find the worst ring seal, and may convince you to go a little further, if #5 drains in an hour, and some of the rest hold fluid waaay longer, or overnight.
Something is funky . That spark plug is telling you a story .