Lemon..... How can I tell weather my oil pump or oil pressure switch needs to be replaced?

One day my lemon (2000 jag xj8l) radiator hose failed and destroyed my speedometer, crank sensors some other things. After I finally finished repairing a whole bunch of parts, now when i start up my car it will idle fine until its reaches proper temp then failsafe cuts on.. OBD reader only sees P0121 and low oil pressure comes up on the display. Also my car never makes it past the 5th check on my OBD refuses to complete the cycle to get a P1111. ????