With an '86 300E with 183K, I am waiting for this to happen to me.

Preparing for the inevitable, I did find rebuilt 103 heads on e-Bay for $795 with no core. I kept the email for future reference: tomdotmebe@aol.com.

At this price, you may wish to consider a rebuilt head (and new chain) given all the work involved in R/R.

Good luck!

Chuck