If the leak is small enough that it doesn't fill the lower cowl and cause running problems, then do not worry too much about it. Set the engine aside for a while then clean the leak area. Dry it with a hair dryer and if you can reach the leak, goop it up with some RTV silicone on the outside to hopefully stop the leak. OTHERWISE: As noted above you will need to dismount the powerhead and replace the gasket. If you intend to keep the engine for a long time then invest the work and replace the gasket.