Is the engine in the 2.5 rs the same as the one in the sti. Also are they alot of aftermarket parts for the 2.5 rs.

for your first question that would be a no, and then for your second one that would be a yes

Well, as for your first question, the easy answer is "no," but the real answer is "yes, but drastically modified for different applications (including modifications to the block itself), so no."