> what sets the voltage and multiplier on the these cards? I don't know of any
> documentation for that.

The core voltage of the MC7447RX1000LB on the Pegasos II CPU card is 1.3 V. This voltage is compatible with the 7448, which can run at up to its maximum nominal clock rate of 1.7 GHz with that.
For the multiplier settings there's inofficial documentation up to at least 1.467 GHz:

* multiplier setting
* localization of SMD bridges
* example position of the SMD bridges

Pin compatibilities from 7447 (to 7447A) to 7448 are not quite 100% for every use case, which might impose implications on such an endeavour (replacing 7447 with 7448):

https://www.nxp.com/docs/en/application-note/AN2656.pdf

In case the target clock rate is below 1.5 GHz, maybe it would be better to use the 7447A (maximum nominal clock rate of 1.42 GHz @ 1.3 V) instead of the 7448. For this it would be handy to know if there were any changes done for the existing 1.4 GHz 7447A Pegasos II CPU cards (beyond the obvious thing of changing the multiplier from 7.5 to 10.5, that is).