The first step is to clean the exterior of the engine, including the cooling fins. To do that you need to put the cylinder head back on (cleaning the mating surfaces first), and put its retaining bolts back in and tighten them, then reinstall any other engine parts you have removed so that cleaning fluid cannot get inside the engine. Next you remove the cooling air cowl (the pressed steel part which has the pull-start mounted on it) by taking out the 4 small bolts that retain it. Put the engine in a large drum or something similar that will catch all the dirt and oil when you clean the engine. First, while the engine is dry, using a clean, dry brush, remove the leaves and grass from all if the cooling fins, on both the outside of the cylinder and on the cylinder head. Then you can use spray degreaser, or a couple of litres of kerosene, to clean the whole outside of the engine. Either way you will need a paintbrush to actually remove the dirt. Dry the engine with rags and inspect it to ensure it is all clean. Post pictures of the clean engine - we will need those to talk about the further steps.
When you have done that, the next step is to see whether the engine is going to need an overhaul. We can talk about that when you have completed the cleaning job.
