Alright, so the tile errors were really abundant in-game, so I decided to have my hand at trying to fix them while keeping your original intentions in mind. I wound up with this [Download]

The water needed to be sloped into , and that caused some errors with the layering and the green bumpers on the sides. I also thought it'd be neat to make it possible for one of the players to dodge the slope entirely and shoot straight off to the East for some trick shot shenanigans, so I removed one of the bumpers.

The arrow on the downward slope didn't work, so I replaced it with a Wheelie enemy, which can give whichever player goes first a speed boost similar to the arrow or booster, but sadly it didn't really work all too well, because there was just too much speed. The green bumpers only work if you're on the ground-- you can pop right over them, and you totally do if you're going fast.

Likewise, I tried my best to get the huge jump to work, but sadly I never could land it once. I switched out the green bounce pad because you need to have already been airborne for that to actually bounce you. The booster alone gives you enough air to go flying over to the East side of the stage, but unfortunately I kept getting too angled and flying off to the South too much. I'm not sure if it'd be possible to make that big gap on such a short corner.

The water near the landing of the huge jump had to go, simply because the game doesn't seem to render things that are "behind" the visible field very well. I tried to keep that part in, but it didn't really work visually.

Lastly, the vents to the UFO were a cool idea, I really liked them, but unfortunately don't actually direct Kirby, and instead launch him over the green barriers easily, which lead me to my death many times.

I really hope that you can make this level work though! It was a super cool idea and I loved the design of it originally, I just can't seem to get the game's engine to co-operate.