You know the ammount of crap you guys keep talking about "sell That POS and buy a GST and get out of HS" is completely stupid and is what started making the mitsu tech page crap before. If you have no knowledge of the 420A motor and what it can or cant do, dont post crap up. This is a Tech page after all.

Anyway to answer your question superkaioken, swapping the 420A motor with a 4G63 is possible but it takes a lot of effort and patience. Something that I would not recommend doing unless you have a lot of time and want a headache. Since you have an RS you can rebuild the motor (correctly!!) and boost it (Correctly!!). Its not easy and does take a little researching and know how but that is your other option and you might be pleasantly surprised at what it can do. Let us know if you need any help.