Air filter and spark plug should change when dirty. Open up both parts and inspect whether they are still clean. According to the manual, spark plug should change yearly, unless you are using high quality part. Air filter should change every 3 years, or dirty. Both parts will cause engine to lost power.
If spark plug seriously clog, or bad, then it won't fire. This will cause misfire. Therefore, it is possible.
These 2 parts are very cheap. Less than $50. Please change immediately
