TDI has a knock

just bought a wrecked early 99 Jetta TDI. It was a light roll over in a ditch and I got it home and tried to start it and heard a loud knocking in the engine and it started and I shut it right off because it was a solid type of knock. I took the pan off and didn't see anything in the lower end that was causing the knock. The rods were all straight. I thought maybe a cylinder got full of oil and bent a rod. When #1 & 4 were coming up on top, it would hit something on the top end that is pretty solid. I used a ratchet on the balancer bolt and it would stop me dead when I'd roll it to the top. When I first used the starter, it would go past it but by hand it won’t. I talked to the gal that owned it and she said it was lying on its top for about 4 hours and the tow truck guy got it back on its wheels, checked the oil and said it was very low, started it and drove it on the wrecker. She said it sounded good then. I pulled the injectors out of 1 & 4 and I put compressed air with the valves closed and # 1 piston will move back down in the cylinder, # 4 won't and I hear air bypassing there too so it looks like a valve in # 4 is the problem. How much of a job is it to pull just the head or is it just as easy to pull the engine? Has anyone heard of this happening from getting oil in the cylinder?

I just came in from taking the head off. I found the # 4 piston top completely ate up and galled to the cylinder wall and head like a cap so when the piston came up to almost top dead center, it was hitting on it's melted material stuck to the head and wall. That's what the material was on the injector too. It looks like I will be taking the engine out. I wish I'd talked to the tow truck driver about how it sounded. The woman said it sounded good but the way it looks, it has had problems for a while.

Does anyone know where you can find oversized pistons for a 1999 TDI AHU block? One guy has them for $995 for 4!!!!!!!!! I found another that can order them from Germany for $125 each plus $40 shipping. Seems very high to me.