If you take the rocker out and can pull the lifter assembly out, the seal is bad and may be missing altogether. Seals are not available separately, but the lifters are affordable aftermarket from INA and have been updated to include a white Teflon/Delrin/plastic seal.
Cost is about $30 apiece and installation with the engine in the car is similar to a valve adjust on a standard 911. It can be done from below on 993 or early cars with conversions.
As of Summer 2020, we are now recommending replacing all lifters on engines with rebuilds and also offering it on used engines.