Body & engine change is legal - you must get 2 receipts from JUNKYARD that the body & engine is part of imported junk.
DO NOT - DO NOT ATTEMPT Plate swap by reworking welding
keep your LC documents intact, LC frame number is on right hand side bone behind front right wheel
though what you plan is Illegal ! BUT I know multiple uniformed (retired) people in Islamabad using Lexus SUV on LC frame and using regn number from 90's
You can take the chance ! too