Sorry, the top frequency of the DS1077LZ is in Mhz, not Khz, but I understand you want to do it simpler.

And comunicating the DS1077LZ with the microcontroller is a lot more work.

Now let's see what specs are needed:

For the Z80 we need the frequency to be double than nominal, i. e. for a 4Mhz stock processor we need 8Mhz from the oscillator, for a 6Mhz we need 12Mhz, and so on. Actually the top frequency of a turbo machine is 10Mhz, but may be Zozo could find some way to increase it, so I think we can't stop on the 20Mhz needed. (I already have 20Mhz Z80's installed on my two Enterprises, waiting for a hack...) .

Then, for the Exdos card is easier, because we don't need double frequencies, only the given 8,9.6,10,13.33333,16MHz

Also it could be funny to be capable of under-clock the processor....

Well. I read some documentations.
The DS1077 has only a divider (range 2..1025). With a basic frequency of 40MHz you will get only 3 values of the set you are interested in (8,10 and 13.33 MHz).
My idea already I verified because the divisor only is not enough to get all the values.
I stayed with PIC (I already ordered). With a set frequency of 64MHz, I am able to generate a signal with a fundamental frequency of 16 MHz (and divided by subsequent integers). This gives us only two values from the set (16 and 8 MHz).
Therefore, you will need an additional multiplier.
I found a programmable PLL circuit ICS501 that can multiply the frequency.
This combination gives you all the value we have (and many others).

Base frequency Fb=16 MHz

16,00 MHz = Fb * 1/1
13,33 MHz = Fb * 5/6
10,00 Mhz = Fb * 5/8
 9,60 MHz = Fb * 6/10
 8,00 MHz = Fb *1/2

Available multipliers are 4, 5, 6 and 8. Dividers from 1 to 65536.
Max 16 MHz * 8 = 128 MHz
Min 16 MHz / 65536 = 244 Hz

Simple is always just an idea.
When it comes to detail, it starts to complicate.
The layout for now (in my head) consists of four main components:
- microcontroller,
- PLL,
- LED display (3 digits),
- encoder (with button).
Is that a lot ?

Easy to do, the microcontroller has an EEPROM.

I can store last value after power off Magic