Leaking exhaust valve cyl. #1 240d

[stephenc1984]

I have a 1978 240d with a bad exhaust valve.
It has over 200,000 miles on a non working odometer and was my daily driver.

The problem started a while ago when I noticed that the car was hard to start.
After doing a compression test and a valve adjustment the number one cylinder had 115 psi where the other 3 cylinders were between 225 and 250.
I pulled the engine and it has been sitting for about a year.

Now that I have time, I recently pulled the exhaust and intake manifolds and sprayed soapy water in the valve ports with it on TDC on the 1 cylinder and applied compressed air through injector hole.
Lots of soapy bubbles seeped out of the exhaust valve.

I then removed the head (of coarse marking the chain and sprocket to keep time With the head removed, to the naked eye, I cannot tell that the suspected bad valve is any different than the other three.
However, after putting more soapy water in the valve port, it leaks through the valve without any pressure behind it other than its' own weight sitting in the port.

My question is what to do next.
Do I replace just the one valve or all?
How much does it cost to have it re-machined and such.

I can get my hands on a junk yard head for 60 bucks or so but I don't know if I should take that gamble.
What do you think?
Thanks in advance!