am i going to loose compression

it i put a stock head off a late 1980 kz on a 1975 z1 900 engine?

also will everything should bolt up fine?

compression wont change.make sure you use the cam sprockets from the old motor since they will be differant than the mkII parts on newer head;)

i dont know witch cam sprockets are witch but i looked at them all (i have 3 sets) and they all looked the same and had the same amount of teeth

You may run into exhaust header clamp issues. The 900 had 6mm studs and the 1000 has 8mm. Easy problem to resolve.

You'll have to do something with the exhaust air passages. Also an easy fix.

Flow wise.....same as the 900 unless it's been modified. No real performance gain to be noticed on a stock to stock swap.

Be careful with the valves. If the new head has OEM cast iron valves, do not swap the 900 valves into those guides. You can use a newer valve in an old [bronze] guide but it's not good to go the other way.