The belt is measured in mm not inches. It should be replaced if it is more than 1 year old. They don't stretch enough to cause what you describe but they do get thin enough to reduce speed. This will NOT happen all of a sudden. more of a gradual loss. The contra spring can be tested on a bathroom scale. The stock spring should take 55-65 pounds to compress until the coils touch. Try a compression test (cylinder). My guess is you lost a valve seat or something similar. Also take a close look at the intake manifold. It is rubber ended,(at the carb) and prone to cracking. Have you looked at the Var?