If the heads are off, then measuring the bore size would be the quickest method of finding the size- assuming they haven't had any unusual rebuilds.
Then check out Tunas site with the displacement table.
http://tunacan.net/t4/reference/displace.shtml
Stock from the factory the 1700 and 1800 came with a 66mm crank and the 2L a 71mm crank. Bore size 90mm, 93mm and 94mm respectively.
Otherwise check the case numbers and compare to this table on Tunas site
http://tunacan.net/t4/reference/case.htm
Cheers
Jeremy