The 420a engine has what I feel is endless potential. Since you dont want to go turbo (even aftermarket) I would advise you to change intake, header, and exhaust... Then cam gears, underdrive pully. You really dont have to change internals but why not just rebuild your engine and throw some nice pistons and rods in it. Also since once again you dont want to go turbo, you can go with a high compression ratio... Dont flame if Im wrong but Im pretty sure that every point in ratio is like 4 percent add to the engine. So I think are engine is rated 140 by the factory so if you up the compression 1 point thats like 6hp so if you took it even higher then that would go higher too (one point wouldnt be a lot you would want to go higher if your not going to boost in anyway.)

I too decided against GS-t on the simple fact of the factory turbo is pointless. I'm not saying turbo is pointless but the factory one is nothing to cheer about. Its not worth the insurance costs. Buy an Rs and get an aftermarket and call it a day, better yet, a week.