Well custom in the sense that you take what's already there and build upon what wasn't possible in its original form. Doesn't necessarily have to mean you start from scratch.
Yes. It's what most hacks use nowadays, dizzys custom emerald engine and skeli's fire red custom engine, which was made when developing unbound then released for anyone to use.
I don't know the lore about dizzy's engine, as I was focused on...
Same with the gba games tbh, but that's mostly because custom engines exist for emerald and fire red, the last hacks for ruby and sapphire were like....royal sapphire, ruby and sapphire cross? And I don't think there's been a leaf green hack...
So you need the hackers memory TV edition to play this? if i recall that's physical only, so what do you do
if you don't have access to the cartridge slot using a SD2VITA adapter for extra space?
I just took a look at it and im not certain what it could be as I've never really messed with this file before, but i'll provide it if you'd like to take a quick look at it.