As already mentioned, its a lean condition that is the cause of the symptoms. Most likely a gummed up dirty carb and pilot. It could be other things, but thats where I'd start.
Any new to me used thumper gets the carb pulled and cleaned. Make note of the jets, needle, and position. Not everyone is capable of jetting... and some previous owners have gotten them thoroughly wrong. So dont assume its jetted correctly. If after you have cleaned the carb, believe the jetting is right, and the problem persists then you need to look for an air leak.
Check the valves. Keep notes if they have moved. Button up and do a compression and leakdown test on it. Dont be aftraid to pull head and jug to inspect further. Again, take notes and keep. Add hour meter.
If you dont know the history and hours on the head, crank, or piston, its worth spending the few extra dollars on gaskets to inspect them. I always keep some with my spares.
Get use to doing these tasks on a regular basis (valve check/compression/leakdown). Even if everything checks out now, you will have a benchmark to watch if anything changes. Be it the valves moving, your compression or leakdown figures dropping.... you will be able maintain and fix before an expensive failure.
Thats my pro tip. Its cheaper to maintain one of these motors than to repair after a failure.
