1990 was the last year of the crossflow 150 GT and XP. In 1991 OMC introduced the 150 60* looper. Post the model number and I can tell you what year you have.
As far as the "leak", If you are talking about it is water leaking, a OMC (Johnson/Evinrude) lower will leak water EVERYWHERE when running on the hose. This is normal. The water pump housing on the earlier engines have a weep hole drilled in the housing to drain the water out after running in cold temps. The housing has drain holes drilled also so the housing will shoot water out as well. A lot of water will leak out of these holes. The later models did not have the weep hole in the w/p housing and will hold higher water pressure because of this reason.
While the engine is running on the hose it will have water running out of it in several places. Between the lower and the mid, the drain hole on the starboard (right) side, towards the transom at the mid, some out towards the prop, and then you will have the "dump" water running out of the prop area. Do NOT put any sealer on this as it will do nothing as this water leaking is normal.
If you have a water pressure gauge, you generally will see around 7-15 PSI at a idle.