Ok how do you know it fouled the spark plugs? Did you try to start it without checking the oil level? If so that is unfortunately probably going to land on your feet not his. Next question - WHY did you pay him in advance? Lawn mower engines are not that technically advanced and are usually pretty easy to repair. It sounds like the majority of it could be cleaned up with soap and water and replace the air filter - and spark plug if it really was fouled out. Sorry to rain on your anger at him but you really have to check it out before you start it yourself after seeing the mess he left. Are you sure he was completed with the work?