A pot to control the sequencing speed is easy, you replace the 1M resistor in the circuit with a 1M pot and 47k resistor in series with it.
Getting "a pitch for each step" is very hard. You need a separate audio oscillator for each step, and each one has it's own pot to adjust the "note" for that step.
For all those oscillators an easy method is to use quad schmidt NAND gate ICs, where each gate plus an RC is an oscillator. The other pin on the NAND gate turns the oscillator on/off with the sequencer.
Any way you do it will still be a large complex project with a lot of logic ICs. How much experience do you have?