Here is how to construct a basic Commodore 64 MIDI interface. It gives the user real-time control over the SID chip via MIDI data in terms of basic synthesis. If you already have access to an Arduino, then this could be quite affordable. The setup (and the outcome) is relatively simple, but it works fine for me and what I need it for. You can use either a MIDI connection or a USB connection with a software-based serial to MIDI converter patch.
via little-scale: How To: Make A Commodore 64 MIDI Interface.