I'll bet your great grandmother would be delighted that you were wearing it anyway.
What about turning the clasp into an enhancer, that could be worn with the pearls, in the front? Then you could get a simple clasp.
Do you know what an enhancer is? The bail of the enhancer opens up so you can fit it over the pearls, then close it so it hangs from the pearl necklace itself as a pendant. I think it would look great!