I'd say the B20F head on top of a 1.8 liter bottom end would result in pretty darn low compression. A rough calculation (I used http://www.dunegoon.org/compression.html ) would indicate it dropping from 8.7 to 8.28, more or less. (I entered the stock B20 bore and stroke, and backed into the cylinder head volume by solving for 8.7:1, then changed the bore figure to B18 size).

Other than that you should use a B20 headgasket to ensure the sides of the combustion chamber don't extend past the metal ring of a b18 headgasket.