Here's a link to a website that has a service manual online.
http://www.xenons130.com/reference.html
I'm afraid having the Prev. Owner do the kind of work you have described may only be rectified by going through the car one system at a time to verify it was correct. I wish I had a better answer, but hang in there.
The oil pressure gauges on these have been notorious for low pressure. The accuracy is more than questionable.
Most of the time it lets you know there is some oil circulating and thats about it. Mine only works when I'm going up a grade of 5% or more. (whats that about..) a replacement nissan sending unit is around $40.00 (ouch)
And that won't guarantee the gauge isn't faulty.
I honestly don't remember reading anywhere where an oil pump went bad, like flat gave out. Not that it can't happen..
I think you'll find most guys who have track/race/modded engines use a stock pump with modified springs and reliefs for either bypass,volume and or pressure.
There is tons of info on this site to sift through with great photo's and explanations.
Whew.. I didn't start out to be so long winded!
Best of luck, Persistance and Patience will pay-off.
Tuck o/