This is my plan as well. I'm just starting out and picked up the small rule book, a box of spiders, and the old brass arachnid. I'm planning on ordering the Ramos set soon, but when I was looking at it I realized I didn't need to buy the BA. Luckily I got it on clearnace so it was only like $5. After looking at the plastic BA it looks close enough to the large spider so I was planning on converting.
For the saw blade i was thinking about using a brass gear, something in the vein of this. It would add to the steampunk vibe while also doubling as a saw like weapon. Planned on just sticking it right into the mandibles of the BA.