They are all the same, but all completely different at the same time. Look at how an ABF head fits on a KR block or the other way round, two completely different engines from different MKs of golf but they sync up and do favours to each other but they both have different valve clearences, different cr, different cams ect.

I guess what Im trying to say is a few main bits will bodge together but other than that its all subtle differences