1800 is about 43 to 45 cc

2000 is ually 45 to 47cc chambers, bar that theres naff all in it.!
so putting a 1800 head on a 2 litre takes the CR up slightly..

valves positions are the same,
very very early 1800's did have a slightly different chamber again, but there all so old now your unlikely to see one..

Stu