I have much the same issue with the stock Zombie Dragon as you. I converted mine all from the Zombie Dragon/Terrorgheist and a bit of green stuff Pictures spoilered below). I don't know if you're any good with green stuff, but it might provide some inspiration:
As far as kitbashing dragon kits goes, the reason why I didn't look for parts outside the box is that stylistically the serpentine dragons
GW historically made don't go well with the beefier Zombie Dragon, in my opinion. Even the wings of the Black Dragon, while no doubt spanning wide enough, aren't particularly long (front to back, not talking about wingspan). Okay for a serpentine dragon, but for my taste not substantial enough for anything based on Zombie Dragon bits or the Stardrake for that matter.
Kitbashing should work with the Stardrake, although I have to say that I'm not particularly familiar with the kit. I don't have enough Sigmarines to justify having one (yet

). The armor bits are a bit of a pain, but have you considered using the Zombie Dragon's skeletal bits to fill in the holes removing the armor leaves? I know you wanted the front intact, but (here's the question about green stuff again) putting bones in there with a bit of green stuff work for frayed flesh and hide might solve that problem.
From looking at the sprue pictures, it seems the armor parts aren't attached to the body (for the most part), so it wouldn't even entail a lot of cutting. I do have another suggestion, though. You could make your dragon asymmetrical and zombiefy someparts while scraping off the SIgmarine insignia on others (if you're so inclined), notch the armor plates around the edges and perhaps add some more damage on the surface, and thus have some rusty old armor plates on the dragon to add to the bony sections and reinforce the idea this dragon isn't particularly healthy anymore. Again, rusty armor (not so much battered armor, I guess) goes against your idea of a freshly slain dragon, of course. Just an idea I had.
Removing the claws from Zombie Dragon wings shouldn't actually be much of an issue. I don't exactly remember how the kit is arranged, but if you cut or leave off the digits, you should be able to scrape or sand down the bony mound quite easily, and maybe need minimal gap filling. Keep in mind plastic is nice for reshaping because you can carve lines in it, so getting a new joint out of the sanded down bit is easily done.