There really must be an oil pressure switch on the motor somewhere, or at least a port (with ba plug in it right now) that it can be screwed into. It should not be hard to find.
Installing an oil pressure alarm is a matter of tapping in a switch... it's just a wee plug with a spring contact in it.... into the port. If the oil pressure is low, the switch completes the circuit to groud (the motor), and the small current through the switch closes a separate alarm relay, feeding a much larger current to the alarm mounted on the engine room wall, or somewhere convenient.
Even if you just had that, it should be enough. make sure it's LOUD. Mine is a car horn, and a relay from under the bonnet of an old BMW. It works fine, now 15 years on.
I also fitted an oil pressure guage... I just put a wee "T" fitting onto the engine oil port (as described), and fitted the switch to one end, and led the oil pressure to a guage on the other end. The guage was mounted on the engine room wall. Cover the oil line with a bit of spiral hose to protect against chafe. The guage was like £15 ($25 then).