have you cleaned out the carb?
Your description of how it degraded sounds like it may have gotten plugged up? I doubt it though since you had a fuel filter. But you describe that the spark is fine so that's where I'd go next. Clean the carb completely and see what happens.
You can usually tell if the carb is a major problem by using starting fluid. With a friend cranking the engine spray a little bit down the throat of the carb and work the throttle. If it runs off starting fluid but dies when you stop it's definitely fuel related.
If it doesn't run at all with the starting fluid test then it's spark/timing
