The fuel still get ignited so there is still a bit of a flame, just not like a petrol engine that puts out quite a bit.
When i test engines after i have rebuilt them, i start them on a bench, i have tried with a manifold attached and others without. There were no flames on either setup. It could have something to do with the fact diesels have quite a bit more exhaust volume compared to a petrol engine.
Most fires are caused be electrical shorts and dry grass in dirty machines. The grass or burning wires catch and burn up and catch leaking grease/oil on fire.
I would throw new oil and filters at it first, then attack the manifold later on.