97 Legacy and an EJ204, what are my options?

new on this forum, so hope all is well posting directly here. i tried looking for similar posts to no avail, and apologize if this is a duplicate inquiry. I have some specific questions.

My situation:

i have my trusty 97 Legacy wagon, it has a N/A EJ20E with 4EAT, i presume it has a stock ECU, and as far as i understand, cannot be tuned, poked or bribed to do any dance other than the factory dance.

now i got a scrapped N/A EJ204, out of a 2008 Impreza (DOHC VACS + 95% sure it has a MAP intake). bought this mostly to waste my time on something and have fun learning about the internals of an EJ boxer first-hand.

but i can't stop wondering if i can put this newer, more powerful and efficient engine to use (all assumptions, seeing as my old 97 is not exactly powerful nor efficient)

so all this is what i have gathered and understood so far, please correct me if I'm wrong, i really want to learn here.

my options seem to be,

#1: replace my ECU with a 2008 impreza ECU that can control the ej204 with all its variable timing goodness and MAF. i'd like to avoid this, seeing as this is quite the process (i get a bit intimidated looking at the wiring mess i have to deal with)

#2: Basically run the EJ204 long-block with my Legacy MAF intake, and install that setup in my car, i've heard i can run a VACS engine without it, is it really that simple?

besides that, would i not need to get my setup tuned anyway? so needing to find an ECU solution as well.

other solutions might be possible, and I would love to hear them (as well as how hard it would be to put the VACS to use). But seems like its either those, or a variation of the options listed above.

my last question;

i want to understand ECU's better, and from what i have gathered; almost all engines (even variants of EJ20's) have their own ECU, some can be "tuned" or have an accesses port added so you can adjust stuff, timing, air/fuel, etc.

So as long as an engine has the same sensors/wiring, it can be swapped/paired with any ECU capable of being tuned? (speaking strictly of EJ20 engines and EJ20x variants).

to close; Im thinking about basically finding a scrap car and taking a tune-able ECU (with the wiring?) and making a franken motor with EJ204 long block and EJ20E MAF intake.

thanks and greetings from Iceland.

Some reading, welcome to the forums.

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