It's pretty good so far, but I feel that the first half is a little harder than the second half (or at least, what I've played). I think this would be a perfect chance to use that Fishing Lakitu from 4-8 in the original game - it was vastly underutilized, and I could see you easily designing a part in the second half based around it.
Anyway, a few things to note:
This part is a bit empty. Throw some coins + a Koopa there or something.
You can just ride the Lakitu down to get this flower, rendering the bridge ultimately useless. I'm not sure of what the best solution for this would be.
Likewise, you can just shoot an egg up there and avoid riding the Lakitu at all. This one is an easy fix though - just raise the platform a bit, or obstruct the opening with cement blocks in order to force the player to ride a bit higher.
Loving the atmosphere so far. Looking forward to seeing more!