I never really tried doing something like this, but if I had to, I would build the general shape with some wire and then proceed to put some blobs of greenstuff on it. Only when those are cured I would fill the gaps with some more greenstuff, possibly with the help of some clay shapers. Finally, if there's a need for that, I would smooth it with several layers of watered-down liquid greenstuff.
One could do all the greenstuffing in one go on the wires, but separating into several steps allows for greater control and lessens the probability of accidentally touching and messing up some earlier part.
I also suggested using blobs because the etheral parts of the mortis engine look more like smoke than fire. If you want to go for something fiery, then I would try sculpting is as described above, if maybe a bit thinner - and then begin adding the flame parts - long teardrop shapes which you could attach to the main 'smoke' part.
Or you could go full hellfire mode and try doing something like Rob Hawkins did on his Nagash
HERE. I didn't find any post on his blog where he would describe how does one sculpt those fires, but maybe you can find it somewhere or just ask him there.
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