do a compression test, then dribble a couple of CC's of oil down each pot and comp test again.

If it gets better - its the rings on the pistons
If it stays the same, its a stuffed valve, or valve stem seal.

Although my money would just be on hardened valve stem seals. Put some oil for "older" engines in it, and give it 500km. If it doesnt get better, you will need to pop the head off and replace them.