While the engine could have problems it doesn't sound right.
Start the engine and reduce the idle via the idle stop screw on the throttle link get it down to about 1000 rpm. Let it run for a couple minutes. Spray some WD40 or carb cleaner around the engine. Spray around the carb mounting and around the intake covers, anywhere there is a seam. If the rpm changes there may be a leak.
After running for a couple minutes, never above 2000 rpm, stop the engine and check the compression. You'll need a compression gauge for this which if you don't have one may be borrowed from some auto parts stores.
If the compression is around 100 psi and within 15# ***** each other, the pistons and cylinders are OK.
Next we will need a spark tester. Put the tester on each spark plug wire, all spark plugs out. Crank the engine and check each cylinder.
Do you have a timing light? We use it to diagnose ignition problems.
Your ignition has both a low speed side and a high speed side, we use the light to check this.
Do these checks and get back to me and we can carry on from there. Sounds like the shop is not competent on these engines.