As you just got the bike I would suggest to give it a complete overhaul should you plan on riding it. First the issue you got on hand. You are right a clogged exhaust and/or silencer can have some bad effects. I'd change the fuel too if it was in there before I got the bike. You never know how old that stuff is or if someone played a bad trick on it, someone unknown to the previous owner even.

Even in a well maintained bike my first look would be at the top end. Some people maintain their bikes just for the eye other are more thorough. On a 125 the top end should be done every 10 to 20 hours for a reasonable experienced rider.

I also would change the bottom end roller bearings, the respective oil seals and the needle bearing of the conrod. If the previous owner has some bills to proof he's done it recently than sure one should take the gamble.

I bought a rm125 a few years back when I started and I was told the piston looks great, it took me about 12km to proof them otherwise. I did the bottom end plus a fresh replate of the cylinder and some other parts I found. I repacked the silencer regreased suspension bearings and replaced those which were bad. New fork seals and cleaned the stearing bearing.

It was well worth it, the bike was great, I sold it to a kid who neglects the bike and just tries to make it pretty.