You can use them for learning, but you cannot legally release a cover song without acquiring a mechanical license.
While these sites provide free MIDI files, you must always check the license. Usually safe for commercial use. free high quality midi files
Importing a complex classical piece or a hit pop song's MIDI file allows you to reverse-engineer the music theory, chord voicings, and bassline relationships. You can use them for learning, but you
: MIDI files that adhere to the General MIDI (GM) standard are your best bet for predictable playback. GM ensures a specific set of instruments is mapped to specific program numbers (e.g., piano is always on channel 1). A GM-compliant file will sound roughly correct on any device that supports the standard, saving you from having to reassign instruments manually. Importing a complex classical piece or a hit
But the old stereotype persists: MIDI files sound like cheap ringtones from a 1999 Nokia. That is only true if you use bad data. The secret lies in finding sources.