a few weeks back i posted if this is possible. well, based on my experiment i would say that mt12 crankshaft perfectly fit in rs12s5 engine. however, these are my findings so far:

starting - no problem
power - the same
torque - almost the same
top speed - almost the same but haven't tried using a temp gun
engine temp - always over heats, though i rich the top needle
plug - using nova turbo no 6

idling - BIG problem! When engine still cold no problem it idles well. but once you i run in the race track after a few laps estimately when i reach half tank, the moment i slowed down and stop the car the engine stalls without hesitation. i tried riching the top end needle several times but the engine still stalls.

it could be in the idle stop screw and low speed needle adjustment........hhhhmmmm.......any thoughts guys???