cylinder head cracked on 6718

Hi, I have just replaced the 2nd cylinder head on my 6718 due to frost damage. The frost plugs had came out of the 1st cylinder head.I checked the oil and it looks like there is water in the sump as the oil looks milky.When the tractor is running,there seems to be a little water coming out of the breather pipe. Did not check the other blocks.

There is a frost plug underneath each head on all pre 1978 engines (on the 9 series a water channel was made in the block so water could circulate there, instead of being blinded with a frost plug) Most probably one of those has popped out and is spilling water into the sump. I'd take off all heads and change all 5 frost plugs on all the heads, clean the gasket surface with sandpaper and reinstall.
Frost plugs rot through over years when water is used with no antifreeze.
Since you didnt overheat it, skimming the heads is not necessary so you'd only need some frost plugs and a gasket set.

Thanks very much for the info,I dare say you will hear from me again! many thanks