Would that work?
My 1541 diskdrive is huuuuuuuge, because it's not just a diskdrive, it has a diskdrive controlling unit and a serial interface.
But I've never seen a 1581, maybe that one's small enough to fit in the c64 case... But anyhoo it'd be cheaper and easier to find a working c64+1541.
Nice hobbying around btw :-)