OK. The regulator part has 21V input. Something a little less than that will be your maximum.
V6 is your output V sensing and initial control stage. R14 is SOT (select on test) resistor for setting output voltage, normally 13.8V. You will have to experiment but maybe you can have a pot (rheostat connected) in here to give some output V control. There is an 11V zener associated with the base circuit of V6 which suggests to me that the minimum voltage might be about 10V (21-11).
Current Limit: R30 is the current sensing resistor and will be a very low value, may even look like just a piece of wire.IC1 will amplify the small voltage developed across R30 and apply this to the base circuit of V6 so reducing the output voltage when current approaches the set point. R23 is a SOT resistor to set current limit. You may be able to replace this with a pot (rheostat connected) to get variable control.
Pots should be wire wound. NOT carbon.
I have no idea what sort of control range you will get by this method. That will have to be determined by experiment unless someone is familiar with this circuit and wants to try the maths.
As thing device is mains powered BE VERY CAREFUL. If possible power this via a 240/240 isolating transformer with sufficient current rating. Use the fuse size as a guide. Unfortunately not everyone has something like this available. I know I haven’t these days. A point to note along these lines a “Variac” although called a transformer is an autotransformer and DOES NOT isolate.
PS. When I was working AWA manufactured a similar power supply mainly for test bench use which had a pot available to vary the voltage from about 10V to something like 18V or so for workshop testing purposes.