quick question about engines

wich one will fit in my original rush : the 18cv-rx (s) or the 18cv-rx . whats the difference why does it have an *S*?

also will the t-15 be a direct fit? no motor mounts needed? and can i be able to swap the clutch,flywheel,and clutch bell from a nitrostar 15FE?


thx in advance
Rick