best idea for the RS is to get either a STi short block.. or a 2.2L block out of a 91-94 Legacy. STi short block will set you back about $1800 or there abouts.. and a NEW 2.2L Shortblock will be about $1450 .. btw the 2.2L is one of the strongenst blocks Subaru has ever made.. i've heard it said that its even stronger than the new 2.5L Sti block.

Put some nice 2.% DOHC heads on the 2.2 and the right gasket to get a mid 7s CR and boost that bitch to 30 psi not a problem

or much cheaper is just swapiing in a WRx engine if you only want about 250-300 awhp. in a 2700-2900 lb RS thats quite enough.