LS Swap Code 4

Im gettin a code 4 with my LS Swap into my 89 DX...(Crankshaft Position Sensor)....What sorta options do I have to fix this. Now keep in mind my exhaust down pipe is just ghetto rigged on there....since Im going to get the exhaust done before the swap is finished......It first I didnt think I had enough back pressure. Now it just about starts.......it sputters and seems rough....I get a code 4(Crankshaft Position Sensor)....Im not to sure if its the sensor....and if I can even replace it. Has anyone ever delt with this ?

Thanks

Brad

ttt