i would first try and clean the area with carb cleaner, sprey point blank range at the crack and the surounding area to get rid of all traces of oil.
then get a new oil sensor and some RTV sealant which is designed to wrok in an oil environment. work the RTV into the crack so it is filled. then put RTV on the sensor threads and screw it in not to tight.
leave it to set for 30-45 min and test to see if it seals. the beauty of the RTV is it will move and expand with the metal expansion.
if it does not seal then try something else. it will cost next to nothing to try and if it seal the sensor might work. i say might work as the GND for the sensor is connect via the threads soo the RTV could insulate the sensor depending how tight it is screwed in. so you might get you oil pressure sensor to work and plug the leak.
here is thought out load, get a stong band clamp and tighten that up anround the boss of aluminum that the sensor thread is in after the RTV is worked into the crack. this should pull the crack back together, befor screwing the sensor. it should help.