Need help finding Kawi NLA muffler

hello! We have a Toro personal pace lawn mower that has given us many years of good service. A couple of weeks ago, however, it started to run a little weaker. Not knowing a lot about lawn mowers, I changed the air filter thinking that might be the problem. But it has progressively gotten worse. Now when it starts, it sputters really bad and when you start mowing it almost always shuts off like it doesn't have enough power to cut grass at all. I have thought that it could also possibly be the spark plug but I don't know. We are planning on taking it in to get it looked at by a professional, but that always costs money and doing it yourself is more fun and cheaper, especially when it turns out to be something simple. Any thoughts and suggestions would be helpful.

Hi Folks, Need some help here. I have toro lawn tractor 16-38 hxl with a B&S OHV engine that when it gets warm, after about 15 -20 minutes of mowing wants to quit. It starts fine. I've replaced plug, checked and adjusted valves,. I thought it was a carb issue and replaced it, so I know its not the carb (24 hours old)or a fuel delivery thing. When it starts to hint that it wants to die, I can stop the blades then the engine runs fine, can even drive it fine. When I think I can continue and engage the blades it wants to almost die quickly sputtering with a bit of popping. SO I press on the parking brake and notice that too saves the engine from dying. Like many now a days money is tight, wife lost her job and I cant afford to have the mechanic fix. I had to throw $300 at it 3 years ago due the ethanol issue in the gas ruining the carb. Been good about using fresh stabilized gas. Could this be some kind of safety switch realted to the blades engaging or the parking brake, that begins to fail when things get hot? I got it in 2003. I've been good in changing oil and all the typical matainenance. I did see a switch that engages under the frame for the parking brake and I dont know where one like this is for the when blades to engage. I notice that when I take the brake off or engage the blades upon starting, that the engine idle changes a bit as if more "load" is being placed upon it. Could his be a symptom of a switch barely engaging? Again things are tight for me and I'd appreciate anyone's thoughts or reccomendations. Thank you.