is it possible to fit a early black top zetec with mechanical lifters to replace a earlier silvertop head which has hydraulic lifters? and is there a differance between the 1.8 and 2 litre heads?

......errr no! I have found this out the hard way!

The difference between the 1.8 and 2.0 is the bore. 80.4 to 84.8.

The chamber in the head is cast to the bore diameter so the compression ratio will be all wrong either way.

The swap of hydraulic to solid should be ok. but not the other way round as the pump won't deliver enough pressure to the oilways in the head.