That's going to be tough. It may be easier to rip out the display from the commercial unit and put your own display in.
A pH Meter measures tiny tiny voltages. While the basic principle is well-known and you don't have to reverse-engineer it, getting it to work "right" with your unit may be impossible. The unit is probably only measuring the voltage intermittently and it may be applying some clever processing to work around the limitations of the physical process. Reversing that processing will be difficult since you don't know what it is.