Some really cool ideas here! The Main Theme has an infectious kind of joy to it - definitely reminds me of some of the opening themes on kart racers back in the day. I love how it transitions into an almost fully directed chiptune composition in the final half of the track. Really nice use of chord arpeggios in the background, and I love how you've handled the percussion. DCPM sounding kicks and snares with the noise bursts for the cymbals and transitionary SFX. The modulated tempo at the end was a nice touch, too!
Straight away, the Level Theme captures the vibe befitting of the title. They always had such upbeat and groovy themes for level, stage and course selections back then and I think you've nailed the feeling here. This track in particular definitely leans more into the tropical side of things, especially in the lead melodies. Gave me some faint memories of Angel Island from Sonic 3!
Finally, the Shop Theme reminds me of something I might hear in a Zelda title. The bossa vibe here is immaculate and I love how the track feels equal parts pensive and upbeat; emulating the player being curious in a shop and weighing up what they can afford. The piano melody here is really great - breaks away from the typical chiptune restriction but adds this whole other layer of ear candy to the track.
Great work - this was a really fun listen, and I'd love to hear whatever else this imaginary game would have in it. I think the thematic execution was perfect and that you really honed in on using your inspirations to craft your own unique take on things.