WHip the exhaust off and have a look at the side of the piston, this can give fair indication of the condition of the motor, secondly roll it over and hold the sides of the blade disc and rock it from side to side, if there is any movement between the engine and crankshaft the beaqrings and seals need replacing.
Lastly, if you have the head off put your finger on the top of the piston and gently turn the motor back and forth by an inch or two, if you feel any time when you turn the motor back and forth where the piston dies not move there is wear in the conrod bearings.
Most times I find clunks are caused by either a blade plate that the hole in the center has flogged out, a bent chassis and the blades hit the deck or worn engine bearings.