Check in this order.
Oil level is correct, top up if not. When was oil filter changed? If there is no history of this replace it, this should have been done at the same time as the rebuild. If oil pressure light still comes on check oil pickup isn't clogged with crap (if an engine has been rebuilt then desintegrated bearings and metal block this in the same way it blocks oil filter and causes low oil pressure) Check that the correct oil is in the car. If it's too thin when warm the pump can struggle when revs increase, made worse because pump is probably old & worn. If none of this works replace the oil pump. As the engine has been rebuilt then on assembly an oil way may have been blocked with crap or old bits of gasket etc which is a hassle to rectify. Good luck!
