As David has (briefly!) mentioned, it IS possible to remove the head and replace the head gasket with the engine in place, but it is one heck of a lot easier with the engine on the work-bench (or even better, a rotisserie). Some of the bolts that secure the shrouding are round the back of the engine and can be a real pain to get at, as are the bolts that hold the rear manifold to the exhaust.