What is considered good compression for a W124 diesel? Local junkyard has one with 320/cylinder. Does a 1987 W124 sedan 6 cyl. diesel engine swap directly into a 1987 W124 300TD wagon? Does it require any modifications?

Five to ten miles after filling the fuel tank, I started seeing exhaust smoke; lots of it. I thought (hoped) "bad gas, water in the fuel". I pulled over, put it in park and hit the accelerator - a thick plume spewed out. It looked white, not blue, with hints of grey/brown. And it smelled burnt, not sweet like coolant. I drove a mile home in a cloud of depression. Next morning I popped the hood and squeezed the readiator hose, it was soft, not hard and the coolant was clean. I started it again, no smoke, and took it for a short ride and saw smoke after 200 yards, then started hearing a clack - more like a metal on metal drumstick. Pulled over, opened hood, no visible issue, but distinct clack from one of the 6 cylinders. Turned off engine, the smell of burned smoke lingered heavily in the air. I got in, started up, smoked a u-turn, accelerated to 20mph, cut the engine, and coasted back home.

Overall, the car is in very good shape. But with 289K miles I expected the head gasket or #14 head to go eventually. But I don't think it has. And I'm very certain that overheating did not happen.

So... what to do now... invest in diagnostics? Which diagnostics could provide value with these symptoms? Meaning: is there a chance in hell that with these symptoms something can be fixed?

Or... proceed directly to swapping in a used engine. (local MBjunkyard offers the 320/cyl from an '87 sedan for $1200 + $500 labor and probably a short warranty). Or do I find a reputable remanufactured engine (anyone know who has one with a non-#14 head? Or are there advantages to rebuilding with my old #14 head (valve job, resurfaced, new gaskets, rings?, etc. guestimate $4k+). Or...

Despondent and optimistic. What am I missing? I'd appreciate hearing from members who have experiences (pos and neg) to guide me through this stinky fog.