Oil pressure sender is on block behind AC/Alt. Bracket.
It has a one wire black spade connector
You will have to remove the brackets to unscrew the oil pressure sender, screw in a mechanical gauge and measure oil pressure, if low then possible bad oil pump, clogged oil galley, or main bearing issue.
Remove valve cover and inspect top of head, if there is significant sludge build up, the oil pump pressure release spring may not be working bypassing oil to engine and dumping it into oil pan, or main bearing issue.
If oil pressure from mechanical gauge is OK then replace oil pressure sender.
If bearing problem, then get a used engine from a 90-92 Toyota Corolla/Geo Prizm and replace the engine.
You can usually get a good used 4AFE for $300.
Make sure engine was from running car.
I have done this on several Toyota's and it is a good alternative to a reman'd engine, which will probably cost you $1600-2000.
Also make sure you get same engine i.E. Toyota made two 4AFE engines, one with EGR (CA. Only) and one without (called Federal/Canada).