Hmm... Are there only four M coins in Sweet Plains 1? I flew all over the level, and couldn't find a fifth. There is a suspicious pipe under a platform near the end, but I can't reach it, and I tried all other pipes in the level, but none of them were enterable.

(Also, tail whipping the jumping fish in Sweet Plains 1 is hilarious.)

EDIT: I've finished the hack now. It was pretty neat trying out those different power ups. I've found a few more things I should mention though:

In Autumn Island 2, the first message misspells "Platform" as "Plataform". Also, the secret pipe leads to that suspicious pipe in Sweet Plains 1 that I just mentioned above (but this wouldn't help me get 5 M coins there, since the game still treats me as being in Autumn Island 2). Fortunately, clearing the goal there still counts as the secret exit from Autumn Island 2. Was that an intentional design decision?

In Autumn Island 3, the first message is the same text from the original game's #1 Iggy's Castle. Did you forget to change that message?

In Autumn Island 4, if you shoot the numbered platforms with Super Balls, they "die" and fall off the screen as discolored fish. Also, the secret pipe in the water seems to be impossible to enter while big, due to jumping too high out of the water and landing on the bridge above while trying to get onto the pipe. Is that intentional, or a design oversight?

And, at the end of the demo, there's a level dot called "Avaible Soon". The first word should be spelled "Available".

Finally, I counted 15 exits (7 in world 1, 8 in world 2), not 12.