4g63T's and 420A's are not easily interchangeable nor financially feasable. Your best bet if you want to turbo one of the motors,

1) check the compression, make sure the motor can handle some boost
2) do some research on fuel system upgrades
3) Don't half ass it. A lot of people who attempt to boost 420A's quit because its more involved that just bolting on the kit. Patience and precise tuning is required to boost this motor.

Since you have 2 there, use one as a guinea pig. Boost the shit out of it and get your tuning the way you want it. If you blow it up, you have another.