First off there are no HEADS plural on a 235 only 1. Secondly, if the valve train is clogged up with crap the rest of the engine is also (wherever there is oil). The only way to fix it right is to pull the engine apart and have it chemically cleaned at a machine shop, followed by an overhaul. You can with a relatively high degree of safety clean it up manually using a diesel/kerosene fuel wash/hand wash/spray wash on many areas that you can reach, however, the crud is everywhere internally.
I would clean everything accessible, fill the crankcase with diesel fuel and circulate the diesel throughout the engine using a hand drill and shaft down into the oil pump after pulling the distributor, this might help clear the plugged up passages that lead into the cylinder head (singular) as well.Drain the dirty fuel out, put in 10w/30 and filter and change the oil after only about 500 - 1000 miles.
