Obviously what you're expected to do is:

1. play the mp3 and record an audio file on your tape deck.
2. input the audio file to your sound card and save it as .wav.
3. do your own conversion from .wav to .mp3
4. burn your own .mp3 of the song on your cd.

Of course you'll need good components and a clean setup to get back to mp3 without losing the sound quality, and by the time you get set up for all that, you could just learn to play the banjo and do your own performances of the song.

The banjo (or accordion, guitar, mandolin, flute, etc.) route may sound simpler, but you'll likely want to add a backup player or two; and dealing with the "available" musicians may require developing a whole new set of social skills. Mudcatters can likely offer a whole lot of advice on that too.

John