If the decompression valve on the cylinderhead is leaking, it's usually because it's jammed open or blocked by dirt and carbon deposits. Open the valve and drown the thing in WD40, keep opening an closing the valve for a minute or two and give it another squirt of WD40 every now and then. The dirt and carbon will come loose and will be washed off by the WD40. There's also some kind of cleaningspray available called "powerclaen", I haven't used this myself (yet) but I've seen it in action, one squirt and a thick layer of grease and dirt will be washed off immediatle. This usually solves the problem of decopression valves, but if it's leaking because of something else (like the previous owner and his obsession of prying things open with a screwdriver), you may have to replace the decompressionvalve or even the whole cylinderhead, it will depend on the damage.