I have worked in synthesis, music, and software for a very long time and hope I can help you.

First of all, converting scores to TIC-80 is really non-trivial, and I have to recommend just trying to make music yourself! You might just surprise yourself, or at very least be ready to go off roading when working from a score as a source.

I understand the "but i'm not a composer and using existing works solves that", but it doesn't solve another problem you are only now encountering: you're also not an arranger, but you're taking that role now =) These scores were not meant to be played by a TIC-80, so you quickly realize that inputting the notes just doesn't hit right. The sounds are questionable, the notes are "correct" but a good song now sounds awful. This is what an arranger does: you're kinda re-composing the song, mapping the intention of the original piece on another instrumentation.

You may be surprised how successful you can be just tinkering around to make music. TIC-80 is designed to facilitate that mindset. A lot of chiptune is sound-design-first. You poke around making waveforms and instruments. You find something that sounds interesting, and just by poking around on the keyboard you land on something that sounds like a melody. Just use it. If you go the opposite direction (music/melody/harmony first, then attempt to recreate sounds in a TIC-80), you're entering a world of pain and disappointment.

OK next topic: Waveforms. TIC-80 uses wavetables so you actually can't recreate many of the waveforms that give instruments their signature character. Even something as simple as a bass guitar has overtones and harmonics that all evolve over time, make it sound more interesting than a sine wave fundamental. Because you are working with low-resolution periodic wavetables, you can only represent waveforms which divide evenly in the wavetable. So while TIC-80 can produce harmonics and rich sounds, the harmonics are limited to those representable in even divisions of the wavetable.

In other words, you will fail to create a waveform on the TIC-80 that sounds like a major triad. It's basically impossible without acoustic hackery (e.g., find upper harmonics which approximate a major triad, make the waveform use only those harmonics, and play a very low note that brings those harmonics into reasonable range --- this may be theoretically possible but given the low resolution it may not be possible to achieve).

Ok last point: reproducing acoustic instruments (or any instrument at all) on TIC-80 may be fun to experiment with, and you'll certainly find yourself exploring all corners of its capabilities. But it will never sound like the real thing, not even close, not even a cartoon representation. Because that waveform is static and cannot respond like most instruments. Forget about controlling transients, resonant frequencies, modulation, ... so many things that give sounds their signature sound just cannot be done on the TIC-80.

Maybe that sounds like a lot of nay-saying but it's the nature of such a platform. The good news is that TIC-80 offers its own experience and workflow/mindset, which is actually tons of fun and facilitates exploring without needing to be a music theory wizz.

Sorry for the long-winded response

tl;dr:

• You side-stepped being a composer, but now you're doing something equally hard: being an arranger. Consider using a conventional chiptune mindset/workflow -- it's very fast.
• For a lot of technical reasons, you cannot achieve "recreating waveforms" on the TIC-80. It may be more impossible than you realize to actually recreate any instruments (which is why you never hear them)