How big a diesel are we talking about? For 13 hp, if I recall my small diesels right, you're looking at at least 500cc of diesel, which would add about a gallon per hour in terms of fuel consumption (diesel) at a constant 13 hp load... as opposed to a much simpler to set up turbocharger system which doesn't add any fuel cost besides the fuel needed to keep the charge cool.
That extra fuel cost could be offset by running the accessories straight off the small diesel, yes (I've often wondered if this was the future of electrics... having a small separate ICE to run the air-conditioning!
), but I don't see the benefit versus complication... would be a cool project though.