I have doubt that your engine seized which means piston is not moving freely. May be first time your dynamo wiring burnt was due to that problem. The engine must not be so hard it is either piston rings or piston head that expanded.
Your mechanic seems to be a non experienced. How come he didn't check the engine hardness. Check your piston and rings.
Second problem could be dynamo will be touching sides of the stator. This can also cause burning of the coil. But in that situation generator will not make normal sound.
Thirdly if you do not check oil time by time then it is possible that your engine ran without oil and made engine sealed.
My advise will be to check the piston head and rings first with a good mechanic because some mechanic intentionally do not repair other problems so that you have to visit them again.
Lastly it can happened that due to overheat dynamo joints open. This happens in cheap generators. People who don't know about winding and joints could not able to detect the problem and when they go to mechanic he tells them coil is burnt. So do change your mechanic and also check dynamo joints and see for burnt coils.
