matth0le
+1y
Input shafts for the tranny are the same (splines and diameter). The Mazda tranny bolts up to the Kia engine like it belongs there. You can even use the Mazda clutch on the Kia 8 bolt flywheel. The Mazda engine mounts bolt up to the FE3 block fine too. There are a bunch of other little things that you will come across if you decide to swap but none are a deal breaker.
Members on this site such as fe3tcourier, madlowmazda, oddballracing, b2fe3 have done this swap with good results. Check for Fred's write-ups on various other sites. He has very useful information. We each did things differently but came up the the end result we wanted - a DOHC, more modern, INJECTED, reliable, inline four in our trucks.
There are a few good resources out there (). Mainly just search Yahoo for Mazda FE3 and tons of stuff comes up. Just read, read, read. Further, don't hesitate to ask tons of questions. I won't mind answering and I'm sure others won't either.
Good luck!
Matt