TOPIC:

What did the small end of the connecting rod look like? Common for the small end to not get enough oil, overheat, gall the wrist pin, and then burn a bunch of oil because the piston cant rock in the bore correctly.

I got the bike from a friend so I don't know how it was ridden before.
Compression was lost during winter when the bike was not fired up for a month or two. Before that it was ok, I did a three week trip last year without any problem.
I don't know what caused valve leak. Honestly, I didn't do any measurementss on the valves. The clearance in valve guide was ok, without any play.
I don't really remember whether there was cross-hatch visible from top to bottom. It was certainly visible on the bottom part where piston rings aren't in contact with cylinder, but if I remember there were some signs of it on upper part also.
I've measured the piston ring gap when inserted in the cyl as the manual suggests and they were bellow service limit. I've also checked the new rings.
The airbox is nice and dry.

Please do interrogate me. Although I'd rather call this a 'constructive dialogue'  

I haven't checked compression now (I have to borrow the meter). It's definetly better than before- the kickstarter has a lot more resistence than before.


@DoctoRot it seemed okay. Also the pin slide out and back in with normal resistence.

PS please do excuse my bad English. I'm not a native speaker. And thank you for all the help 

At this point I don't think there is anything you can do other than remove the head and cylinders and check everything out. is it possible an oil ring ends got overlapped?

I agree that head removal and thorough inspection in almost inevitable.
Hmm if I remember correctly, the oil ring is a slotted type. Can a slotted oil ring overlap at all?

I forgot the 750 twin use a one piece ring, it cannot be overlapped.