stumped on my 86 200 sx

i have an 86 200 sx 2.0,i picked it up cheap and am restoring it,i couldnt get it to start on its own but it ran fine while spraying starter fluid.its got fuel preassure going into the intake but its as if its not taking it,so i checked the ecu for codes.it threw a 31(lead signal)then i saw right in front of me where it said 84-85 models only.hhmmm.i just so happened to have a spare ecu hanging around that said 86 only,great!except it has 3 prongs and car only has 2,whats going on?!