1 part molasses to 3-5 parts water
(depending on the extent of rust). Soak a day or so remove, rinse with water, inspect, put back in...you will soon get a handle on how your mix is working, several days, a week maybe or more.
The head I built up for the 1293 was hot tanked by the machine shop twice...I was not happy. A week in molasses/water, it was clean.
Head got skimmed face, hardened exhaust seats, new bronze guides, new valves/duplex springs...Remember
- all grease/oil etc. must be removed first, paint too if overtop of rust.