White smoke when up to temp. Any advise greatly appreciated!

Hi everybody. I have just aquired a barganous '83 Mk1 Golf GTI Cab.
(i swapped it foir a gearbox from a mk3 16v i broke for spares that i bought for £100!!!) Body is really straight and pretty solid, has dull paint, roofs knackered etc….
Whilst checking it out have noticed that when running during warm up it runs clean (slight misfire when cold) but once up to temp smokes white smoke pretty badly (whilst running well on 4). Temp gauge reading is good (approx 1/4 to 1/3rd way up gauge) Engine is showing 102k miles. There is no oil in the water, no water in the oil and absolutely no sludge in the rocker cover.
However the engine does look like its been kept in an oil filled pond for years as its filthy, probably never been cleaned.
Has anybody got any ideas.?
All info much appreciated…

Is it definatley smoke and not steam, and is it using water?

I had this and the break in the head gasket was from the water gallery to the combustion chamber, so never got any water/oil mix only lots of steam  :dontknow:

Sounds like water is getting into the combustion chambers - either head gasket or cracked head I suspect

Cheers guys guess i best pull the head off and have a nosey then. My bet is its cracked as the engine is minging already. Got thick black residue on front of head and it seems to boil in places when engines warm so must be water escaping…
Shame i junked my spare engine about 6 months ago!