Have you pulled the head? Depending on your mileage, you may want to call up some machine shops. I was priced out $500 labor + parts to have my GE head pressure tested, seals replaced, valves checked and cleaned up, etc. Depending on if anything needed to be replaced ( bad valve, cam, etc. ) it was going to cost around $700 - $1000 to have it rebuild and made new again.
If this is too much, I would try to find a local buddy to help you out in replacing these valve stem seals. Most shops will hammer you on the cost to do this, and will probably charge you more than the $1000 rebuild cost above if you removed the head yourself.
Is this a daily driver? You only ride?