as you are saying your xr3i is from 83? am i right? then a never gearbok from 86- won't fit right in as i am told, but it will fit with some modifications of mounting brackets i think. the easiest is to find a xr3i or 1.6i from 80-85 as they will fit right on.

your other question about the different in the engines. hmmm what can i say? the 1.6i from 86- has ke jetronic but the xr3i from 86- has k jetronic. thats the main different. some modifications are maybe bigger nosles or so im not sure realy