Come to think of it there are two sealing rings; one between the tensioner and the cylinder head and one between the sealing plug and the body of the tensioner (between the small hex and the big hex).
If the leak at the head interface, then replace that seal. You'll have to prime the tensioner as described in the shop manual before it goes back in the head.
If the leak is between the small hex and big hex, the tensioner will have to come apart. I wouldn't do it in-situ because there are spring loaded parts waiting to get lost behind the sealing plug. The manual says nothing about taking the tensioner apart other than that one of the internal components was upgraded in late 87.