I was shown a recipe for sweet and sour by a student from HK while at uni. It was basically the syrup from a tin of pineapples, heated and slightly thickened with corn flour, and then add tomato ketchup to provide the sour taste and colour. I would imaging that Kung Po is the same with added chili.
I have my own sweet/spicy Kung Po stir-fry recipe.
Really quick for during the week without buying the manky pre-made stir-fry sauces:
Add some sugar and boiling water to a frying pan then chuck in fresh sliced chilies, dried chili flakes, powdered chili, honey, paprika, salt, lemon juice, dark soy, rice vinegar and a touch of sesame oil.
If it's too thin, add some Arrow Root to thicken (like cornflower but goes clear instead of creamy) if it's too reduced add extra water.
What type of sweet and sour are you after?