Also check your crank shaft. If you see scratches along the intake opening, that means the bearings are bad and the crank shaft is rubbing the engine case. If that happens, your engine will run hot and/or losing power. And sooner or later, your piston and botton head will show you the sign of detonation (the surface is rough, not shining, looks like sand paper), and your engine will start eating glow plugs.....

Anyway, I suggest you also check the bearings, since your engine is not brand new and has been used for awhile.