I’m sure that you're talking about this cluster sprite. The files are 'flowers.asm', 'flowerspawn.asm' and 'flowerspawn.cfg'. I don't know how to insert these sprites, so try looking for a Cluster Spritetool tutorial.
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First part of the level is a grassland and the second part is a forest.
UPDATE: I found a glitch with the Yoshi Coin in the bonus area. I also changed some graphics.
UPDATE 2: I found another bug in the forest part near the end. I also enabled vertical scrolling in the forest part.