Drain pineapple, reserving juice. Add water to make 1 cup. Stir in sweet and sour mix.
In a large skillet sauté onion and bell pepper in oil over medium heat, stirring constantly for 4 minutes. Add Spam and water chestnuts and cook for one minute longer. Add sweet and sour sauce and pineapple. Bring to a boil, and boil 1 minute.