Here is a list of "some" of the s*** you will need...
G-15M-R Manual transaxle, clutch, flywheel, cluth hydraulics; pumps; forks; bearings; etc. MTX shift linkage, shift lever, shift knob, interior trim for MTX, shifter housing, shifter bushings snap rings and cup mounts, 3 pedal cluster replacement, MTX damper mount (#4 engine mount I believe) and possibly lower subframe support bracket , Master cylinder (not sure if entire thing needs replacement, or if the ATX's brake master cylinder is the same as the MTX's), possibly the MTX's axles and half shaft, all the MTX's sensors (not many), All of the MTX associated bolts, studs, and misc hardware, 3 qts of gear oil...To complete it as if stock you would also need a MTX's instrument cluster, being that it lacks the Auto's PRND12...
It can be done...If you go through a dealership for all of these parts new, you will easily part with 5 large...If you can find a used 2.0L MTX protege you will be much better off...FWIW I am not sure about the ECU...You are correct that ECU controls the automatic transaxle, and I don't know what kind of errors you wil run into when all of the sensors wiring incorporated into the harness no longer "sense" anything....
also remember, if you have an honest 270whp, beefing up the drivetrain with thick-ass axles and an LSD you will snap that aluminum case faster than our economy dropped...the LSD is a must, and if you leave the gearbox stock internally try keeping the thin axles...snapping one of those would be a lot cheaper potentially than another gearbox...