Thanks for the hint. The next thing to try would be to connect the tape head directly to the IO-Pins of a Raspberry PI and then use a simple manchester encoded

Thanks for the hint. The next thing to try would be to connect the tape head directly to the IO-Pins of a Raspberry PI and then use a simple manchester encoded signal ramping up the frequency. A Raspberry PI can push up to 1 MHz and has an internal Schmitt-Trigger which would be perfect for reading the data back.