You're in luck — I believe that tape reading and writing is already CPU speed independent.
For reading, the 1-bit tape input is provided as the CA1 input to the keyboard VIA. The OS sets it up to generate an interrupt on its rising edge. At each interrupt it interrogates the VIA's timer to determine how long the most recent wave was.
So as long as you don't change the VIA clock speed, the CPU speed shouldn't matter.
See the routine from
LAB_F98E in this disassembly which, as it purports to be, is the routine called by a relevant input-level transition interrupt, and interrogates the second timer of the keyboard VIA, VIA 2 in order to discriminate the wave just seen.
I'm a little vaguer on it, but I believe the same mechanism in reverse is used to write bytes — looking at
LAB_FBF5 and calls into it, the procedure seems to be to toggle the output and establish a VIA timer interrupt for when the next toggle should occur.