Things that went wrong with some CT/XL/XR farm bikes I have had that may be similar to a postie,
-blocked drain/breather hoses on carby. They have similar symptoms and its usually those bloody little wasps that build nests up the hoses and block them up. Try blowing them out with an air compressor.
-blocked jets, same with any carb, remove and blow them out.
-another great one but un-related is the slide needle wears out and they won’t idle.
-possibly the coil as well. They can pack it in and give similar symptoms.