Well you don't actually need real instruments I record with my MIDI keyboard, but it all runs through various VSTs anyway to emulate different instruments. In fact, you'll find most people here don't actually play any instruments, everything is computer generated by just writing midi tracks in a DAW. The soundfonts and presets in FL will be a good start, but then you can always obtain additional VSTs in the future to expand your musical library.
I wouldn't try to think that the song needs to fit in a Sonic game. Music can be reinterpreted and arranged in any way you see fit. Even your mix for example sounds drastically different than the indie rock original version. Just try to think about different ways you could interpret the source tune in your own style. Just by listening to your vocal mix here, for example, I could imagine slowing down the tempo a bit, adding in some driving piano harmonies to accentuate your vocal layers, maybe some held strings to add a bass presence, and maybe a bit of hand percussion for beat/time-keeping purposes. I'm not sure how familiar you are with all the music here on OCR, but a lot of our remixes sound quite different from their originals.
If you are new, take a look around the Music Composition & Production forums. They have a lot of helpful topics from really helpful people on how to get started on all aspects of music production. It can be really beneficial, regardless of your level of familiarity with VSTs, DAWs, and other random acronyms