Air cooled diesels are noisy but there is plenty of sound dampening material around, 50mm or 2 layers of that and a fibre glass shell around the engine is the plan .I'd prefer a light weight water cooled diesel but apart from being very bulky and hard to come by they are heavy and expensive !
As an aside it is possible make a diesel quite quiet, comparable to a petrol engine by injecting propane into the airbox. it has the effect of lengthening the moment of combustion.... the reason for the diesel 'clatter '... you might note how quiet modern common rail diesels with their multitude of tiny injections of fuel through the combustion period are. Direct injection diesel have a very rapid burn, it's quite efficient but give the characteristic loud "crack".
Do NOT spray propane into the airbox until the engine is started .
