What is the usual 'cause' is lean burning resulting in what is called 'crank case combustion', some call it sneezing engine for short. The combustion on top of the piston goes by the intake ports and continues BELOW the pistons. Might be due to faulty thermostats, clogged carbs or whatever gives a lean mix. Sfter some time the Gel Seal between the two halves burns out.
You need a torque wrench, good socket set and some general tools, but not much more.
Parts you may get from this site, Marine Engine, BRP dealer or any Sierra dealer.
In addition to the gasket kit you need a tube of Gel Seal II from a BRP dealer and I recommend a can of Sealing Compound as well.
http://www.marineengine.com/parts/j...rer=Evinrude§ion=Cylinder+&+Crankcase
However perhaps the most important is an OEM (original) Workshop Manual which I know Marine Engine has. No Seloc or online!!!!!!