4g63 N/A came with 135hp I think, 420a N/A came with 140hp.
4g63 is a Mitsubishi Engine while the 420a is a Chrysler Engine.
Although the 4g63 has 5hp less it can produce a lot more power and handle a lot more abuse. Not to mention they sell more parts for it then a 420a.
I've heard of N/A 420a's when fully built going 14.x in a 1/4 mile... don't know about a 4g63 N/A.

If it were up to me I would go with the 4g63 N/A. I've owned both 4g63 N/A and 420a N/A, the 4g63 N/A felt like it had more power and both had similiar parts installed.

Just make sure you buy one between 90-first half of 92. They came with a 6 bolt motor. If you find a 92 I would run the vin to find out if it came with a 6 or 7 bolt.