Ian, sorry for the long reply.
I have not built this circuit.
You have misread the diagram, the input of the 7805 is connected to
the power supply (pin 1), the output (pin 3) goes to the logic and
Do not use 24 vdc on this circuit, you will spend much time
repairing. Reason; First, the input zeiner, does not show a spec.
Could be over-voltaged at 24vdc and it is only rated at 3 amps. 7805
regulator is a limited power handling capable device as a TO-220
package, unless you get it in a TO-5 configuration, at 24 vdc you
will have a potential of 19 watts developed during regulation, the
device is rated at 15 watts max and you better have extremely
effective cooling getting near 15.
I/O is rated as RS 232 levels, if you float ground, plan on buying a
new computer, you will burn out the RS 232 port and whatever is
connected to it, simultaneously with the logic circuit in the
diagram. Using a beefier FET will buy you nothing as everything else
will be dead. Additionally Q5,7,9 and 11 would have to be re-rated
for the installation, they will more than likely expire at the first
kick of the servo under load.
Use the circuit as is, 15 VDC at the input with a Max of 8 Amps will
supply 120 watts and spin this device with considerable force, you do
not need to go to 24 volts to do so would require a major redesign of
an otherwise good circuit.