Briggs and Stratton engine help

Hi guys. Here's the deal. We have an old Hechinger riding mower that we've been using at our home for years. Its been here since I can remember. It used to be our main mower but over time we eventually got a nice zero turn for it cut much much quicker than the little Hechinger. However, we still use it in the fall with our tracvac to suck up the leaves. This year though something took the turn for the worse and it began running very poor. It would be decent one minute then it would bog down, barely be running and then it would pick up again. It would do it 5 minutes later and eventually it would just shut off. My father took the carb apart and replaced the fuel filter and pump, to no avail. It would try to start backfire really really bad and then shut off. He already has purchased a new huskavarna so this little tractor is doomed unless I fix it. I enjoy working on my 86' pickup but never got into real engine work on it and figured that rebuilding/working on this little engine would be great experience to learn on. Besides if I can get it running i can keep it and mess around with it on our farm. Also it was the first lawnmower I ever drove lol. We had to disable the seat safety switch cause when i pushed the brake i had to get off the seat to reach. Any help appreciated.

oh yea, the engine is a briggs and stratton 2 cylinder 18hp i believe.