Nice game! I really enjoy the concept, especially limiting/enabling the player's controls that way.
There were a few bugs I think... (I played in Emulicious) a couple times fighting Bowser he ended up not firing at me, I guess I got too close and he got stuck not knowing what to do, even if I backed up? The pedestals also trigger multiple times if you don't walk away(getting the boomerang was especially buggy, the boomerang itself stopped working on the island at one point), and in the end game I had to try to quit the game 8 times before it actually quit (to the point I thought part of the ending was going to be not wanting to leave lol)
Also still not sure what the item in the bottom left was... although that was a fun little segment trying to get around the boomerang!

None of those were too troublesome though, it was still a fun time!

The general idea is pretty good, some music and/or SFX would have been nice.

As someone who also made a game about assembling a gameboy (or "JamBoy", if you will), I was excited to try this out! I see Robert mentioned the black space (leading to the  A button) a little hard to find, but I thought it was designed rather nicely with the larger space on top telling you to find another way in! An important distinction though, I played on browser, not a DMG.

Unfortunately though, I couldn't make it past the boomerang bros north of the castle. I would kill them and die when walking through the path by what seemed to be an invisible boomerang of their's. Also, I don't know if this was intentional or not, but I would also have to regather the B button to use the boomerang every time that I died.

Lastly, I was unsure on what the item in the bottom left was. Perhaps it's explained later?

I like the idea a lot! Limiting the player to their abilities until they gather more buttons takes the metroidvania style design to a new level! I would love to see an more with not only an update to the boomerang bros thing, but to expand more on the limited abilities idea - such as making more challenges like the initial only-using-up section but really exploring all of the different ways you can build that out.

I managed to beat the game and my Gameboy is now assembled! Really neat way to introduce people to the Right to Repair movement!

Well done for making a playable Game Boy game, all of this feedback is based on me playing it on a Game Boy DMG. I really enjoyed the start, it seemed like I was going to enjoy it, but after I collected the D-pad and entered the large open space, most of my time was spent looking for something. I gave up after I collected the A button. To get to that place seemed so random, it took me forever to find that small black mark on the right of the map. Before that, I collected the thing on the pedestal, but it did nothing except make the game slow down. There was no music unfortunately, so it was a frustrating and silent experience that I couldn't finish. Once I collected the A button, I tried the gate and the boat but nothing happened so I'm sorry I couldn't finish it. I hope this is some useful feedback and will help you in some way.
However, I love the idea of collecting parts for your Game Boy and I liked the game up until I felt lost in the large open area.