If the engine is severely damaged and won't run, look for cracks or holes in the block. Take a good look at the air intake system. Start the engine and observe any blue smoke, engine noises and lack of power. All of the above can give you a good indication of what you will find when you disassemble this engine. If the engine does not smoke badly or make unusual noise, but lacks power, it may only need a valve grind or tuneup.
Carbon buildup can be a real problem on gensets, pumps etc.
At this point you have decided the engine is smoking and needs a rebuild. I like to start by removing all the shrouding and use either compressed air or a pressure washer to clean the engine thoroughly before opening it up. You may want to take it to a car wash. This gives you a clean unit to work on. Leave the air filter on for this and duct tape the exhaust outlet and fuel cap vent.