The black or brown stuff on your motor is very likely dried castor oil. It makes the motor and muffler, etc, quite unattractive to look at. It may be removed easily by placing the affected parts into a "crock pot" or similar electric heated slow cooker. The pot is to be filled with auto anti-freeze/coolant. NOTE: YOU MUST NEVER USE THIS COOKER FOR EDIBLE FOOD AFTER THIS. ANTI FREEZE IS A POISON. Cook the motor parts overnight, preferably outdoors where the fumes will disperse and cause no illness. Keep out of reach of animals because they are interested in the taste and will die from it. Be aware that some plastic or rubber parts can be damaged in this process so some disassembly may be required. The black stuff burnt onto the piston and/or cylinder is the same castor, but it will not hurt anything..if fact, it actually provides additional lubrication for the motor parts. The process is easy, but take care to do it safely.
best of luck.