Step 1: Don't ever listen to anything your "home boys" tell you about Miatas or turbo's ever again. Just don't.
There is nothing wrong with the stock ECU. There are lots of guys who run well over 200whp on the stock ECU. Guys around here run an Emanage Blue piggyback to well over 250whp... so the stock ECU will take you very far.
We can help you out, but we need SPECIFIC INFORMATION!!! Make a detailed list of everything you currently have assembled for your kit, including the specs on the individual pieces... what kind of turbo, what size is the IC, etc... If you "don't know ****" about turbo's, you should probably stop spending money until you DO know something so you can make good purchases.
Some more stuff to tell us about.
What is the car going to be used for. Daily driving, autocross, drifting, drag, etc...?
How much money do you have?
What kind of power do you want?
What kind of tools and mechanical skills do you have?
How many miles on your '90 and what is the current condition of the car. PLEASE list all MODIFICATIONS you currently have, like tires/wheels/suspension/brakes/chassis/etc...
What are your compression numbers?
AND NOW YOU NEED TO GO HERE AND READ THE WHOLE THING!