this is a common problem among hondas at 1990.
i own a 1991 honda accord. recently i got the lower spark plug tube or "tower" seals replaced that are under the rocker arm assym. they wernt even rubber any more, they were plastic...i had no leaks at the time but i knew it was bound to happen.
since ur there, put some new valve cover seals with the upper spark plug tube seals as well and possibly use some black sealant for them.
check all the seals even by the distribitor. since u have that cam shaft out with the rocker assymbly u might as well replace that o-ring for the distribitor...and possibly MORE LIKELY the cam shaft seal which is prone to leakage.
i did all this, pretty fun, time consuming but really u only do it once or twice.
cost wise at a shop, around 110 for the cam shaft seal
maybe about 140 for the lower spark plug tube seals...look around, u will get a valve adjustmetn too.