Where we had trouble with smell of gas and clicking but no ignition was the metal screen with the little holes in it over the orifice outlet and under the ignitor, was all rusted out. Got a newer "burner" with the screen ok, and no problem with ignition.
Maybe not the same set-up as Doug is thinking of that needs a complete pull-out, but with ours you can get at a lot of it in place.
I had wasp nests plogging things up at first, but that cleared up by just cleaning the exhaust pipe. I can get the burner out, but do have to disconnect the gas line. It is awkward to get it out, but it does come out.
I did see a youtube where they had to wash out the wasp nest dirt way in back with the furnace out and hosed it all out, but thank goodness, I didn't have to do all that.