Oils seals are intended to allow a bit of oil into the guides to allow some lubrication. When they get old, they tend to allow too much from the rubber deteriorating or just hardening. When this happens, the bike smokes. The smoke is coming from excess oil in the combustion chamber. While it may be technically possible to change the seals, I believe the head should come off as the combustion chamber will be fouled with oil and should be cleaned and the valves removed and re-lapped to ensure a good seat as the crud can get in there and ruin the seat and hence compression which in turn can cause poor combustion and even more oily crud. this is a dandy time to check your valve guides as they may or may not need to be changed out. If valve guides are swapped out, I think you should always recut the seats because the valves won't necessarily hit the same exact spot on the seat. I have rebuilt a few cylinder heads and never have seen one where the seals were shot that didn't need the clean up I noted. When you take off the head, re-anneal the copper stud washers and buy a new head gasket.
