If it was running good then suddenly went to "trying to run, but can't quite" maybe some old crud in the carb or the tank or fuel lines migrated to the jets.
Could be that the idle mixture screw is not optimally set.
Or an air leak has started, maybe where the carb mounts, or at the gasket where the intake meets the cylinder, or the cylinder base. If you could get it running in some fashion, you could test those places by spraying some carb cleaner on those areas and noticing any changes in how it runs.
Or, more difficult, it could be leaking seals on the crankshaft.
Or it could be low compression. You should be able to get to 120 psi. Normal new would be about 140 psi.
Weakness can also be due to a plugged exhaust, but that comes on more gradually. (edited)