Stock gearing is only really appropriate on a stock or near to stock ZA50. A pipe on a stock cylinder is probably ok with just a taller front sprocket but anything more I'd continue reading... The gearing you choose should be related to how much power you are putting through your bike, and your riding environment.
I have used both 19/38 gearing and 20/35 gearing on two bikes. What's important is that you know what RPM you are spinning relative to speed. The best way to do this is play around with the gearing calculator here - https://toolbox.martysgarage.info. You can also purchase a digital tach that can read out your RPM. The general consensus is stay below 9000rpm. Maybe even 8500. All the blown ZA50s related to crank failures or bottom end failures I have seen were spinning like 10,500-11,000 rpm. Less of an issue with rebuilt motors with a new crankshaft.
Take your situation and apply it there. Are you riding primarily in the city and know you are not hitting your top speed often or for prolonged periods of time (passing is OK for example), I like 19/38 or close to that. Riding more open streets where you might be sitting 40+mph more often than not? Definitely go with a 35 in the rear and probably the 20 front.
Another thing to consider is how the taller gearing works on the ZA. Taller gearing lets you ride out your first gear longer. My ZA with the 20/35 gearing shifted a good 4-5mph later than the 19/35 bike did. This is in conjunction with a flipped 2nd gear clutch to be clear.