are mk1 2 and 3 cylinders the same?

The powervalve on a Mk2 is slightly different to a Mk1 in that the Mk2 the powervalve does not restrict the flow out of the exhaust when fully open. I don't believe there is any significant difference between Mk2 & Mk3. All are interchageable.

I have a Mk1 engine with a Mk2 barrel and an AR head.

thank you very much :-)
thought of buying one and tune it a little... but want an original one if i do anything stupid ;-)
or it starts using double the gas...
i use my mk3 to work every day... and i do have other toys :-) but i can`t help it i just need to do something with it. ;-)