yes they are, the early aircooled models are the best, no water to help put the fire out and shorting out the starter cable is a good way of lighting them up, when this happened to mine i was able to get hold of a fire extinguisher, but i rekon if you left it, it would go up pretty well. i think you'll find the de-ville is the best bet as the fabric sunroof burns well and the plastic bumpers are obviously a better prospect than the chrome bumpers. I am not sure about the FSM models, do they use a starter cable or do they have a key, if a key you may have to use an external source to ignite your 126, although it may be possible to rig something up with a spark plug and a bit of petrol.
I seem to remember that someone posted a link to burnt out car in poland as further proof that 126s are flamable