According to wikepedia (in Dutch) there's a turbo version of the OM601 fitted to sprinters - but I'm guessing they are as common as rocking horse droppings.
Mercedes-Benz OM601 - Wikipedia
I don't know why the information hasn't been transferred to the English language page - there's a lot less information there...
...anyway thank you both very much.
This guy on BW is saying he just fitted an OM603 turbo to his 1.8 litre M102 and now he can go back in time =>
My 190 1.8 TURBO - Mercedes-Benz Forum
No pictures of the engine bay just yet. Two videos full of hissing and giggling - and there's another 1.8 M102 turbo conversion on youtube too - but again no engine pictures...
...mention of similar goings on on an English W201 site though.
I'll make some measurements of the M102 head tomorrow - thanks so far