How does he start the process where he left off using this approach?

He's made it clear enough that he wants to have different access times for "ROM" writes and ROM reads. That's why he wants to change the oscillator. It's clear he needs an external clock driver and some logic to make the frequency change when he's writing to his "ROM."

He's also got this goofy notion that he can do this with a 12 MHz crystal timebase. I'm not at all sure that's reasonable.

RE