welcome! Always amuses me how many people sell G40s and then buy another one because they miss it so much.
You can fit a 22mm master cylinder but it wont make much of a differnce, just reduce pedal travel. More useful when you've put bigger calipers on.
I'd probably start by giving the brakes an overhaul, they aren't that bad when they're in good order. New brake fluid, decent set of pads. Also strip down, clean up and rebuild the rear drums, these often get neglected but will make the whole braking system crap if they're not working right and adjusted properly.
I guess also make sure it actually has g40 brakes on it too I guess, these are still 239mm but with vented discs. The non-vented ones overheat and fade really quickly.