Clutch kit part # 19020051A i fitted this kit to my 750 , no problems as yet

I'm not sure but I think that o-ring is not standard and you'll need an OEM spare part if you find some oil leak

you mean clutch fluid leak?
I`ll try to find one today.

and what about bearing? Could it be that I have only pin with bearing balls?
without any other bearing?

and, what I have noticed - the level of pin, and the level of piston are the same,
shouldnt the pin stick out more? (as it take all power from pressure plate)

no, i didnt mean spring under the piston
at the top of the cover, there are 2 (screw-)threads
one for drain, and another for supplying clutch with clutch fluid
and this one was damaged, previous owner instead of repairng
scre thread, just put there a spring

but anyway..

here are 2more photos:
first - after removing pin, and the second after puting the pin

same piston issue in thie thread
viewtopic.php?f=1&t=3278&st=0&sk=t&sd=a

Anyway, I can see the outer ring of the bearing (or part of it) still fitted inside the piston.
If you can't pull it out, try to cut it with a dremel grinder, taking a LOT OF CARE not to damage the piston!
Then wash it carefully with nitrous cleaner and grease a bit the seat

just going through the parts catalogues I noticed that that piston shown above is the one from the 906.