A few tricks I learned from an old mechanic.
drain the oil and refill with ATF. start the motor and let it circulate briefly. Let it sit for a day, and start it again. If you can let it sit for a week and start it every day for a few seconds it works best. The detergents in the ATF will clean away sludge and grime. The drain it all and change the filter. Add some Lucas oil treatment or Seafoam when you refill with oil. Drive it for a few hundred miles and change it again. This last time i recommend using a small particle oil filter, like Mobile1. After that you go back to your regular oil change routine.
this works. i have done it with old cars and bike i have bought for parts or fix and sell.
Another thing he told me that I haven't tried is to use 2 quarts of engine parts cleaner solvent. Add that with a new filter and circulate a couple times, let set and change the oil and filter. That should clean out anything in there. Like I said, I haven't tried it so do this at your own risk.
It beats throwing a handful of sawdust in there. He said they used to do that too if they just wanted to get rid of the car.
It won't hurt to try these. it cheaper than rebuilding the heads.