Using only a computer and a tape recorder you can copy the old datacasettes on to your computer. Then you can either distribute the information on the datacasette or copy it on to a new tape.
Step 1: Supplies
A tape player/recorder
A male to male mini sound cord
A Commodore 64/128
A datacasette reader for the Commodore
2 cassette tapes (or 1 cassette and 1 tape adaptor)