I am peicing together a OBD1 B18B and it's ready to be put in but I have one plug that is open and I'm not sure what goes here. It is the plug just next to the oil pressure sending unit to the left of the oil filter ****. Thanks

 

That plug on the right side of the oil filter location is suppose to be on the left side also. The person before you modified the left side for a catch can setup.

 

^^^ is correct you will need to replace it will the same as on the right or run some sort of catch can set up or install some kind of plug it looks like the hole is tapped

 

could I just plug it with a bolt or would that be harmful to the motor?

 

It is a threaded hole.

 

catch cans are a smart idea. since you're already tapped, set one up. the pcv system just sucks for high performance use and especially boost.