Aside from the issue of a proper fit, can you think of any reason why a Fibrament stone might not work?
To the best of my knowledge, all of the Bakers Pride countertop ovens dedicated to pizza, or pizza and pretzels, are electric and use stones made of Cordierite, which is a well known material whose thermal properties can be ascertained. By contrast, the material used to make the Fibrament stones is a proprietary composition, whose properties may not be readily ascertainable. In fact, the company often jokes that if you want to know the composition of their stones, you will have to buy the company. So, it's hard to say how the Fibrament stones will perform in a Bakers Pride countertop oven. However, I have read that Fibrament stones are used in at least some Blodgett and Garland deck ovens, and that Roto-Flex considered using their stones, so there is some history of use of the Fibrament stones by some of the majors in the pizza business.
My recollection is that one of our more recent members, Otis Gunn, who makes an occasional appearance on the forum, replaced the stones in a Comstock Castle oven that he uses in his mobile pizza trailer with Fibrament stones. I believe his Castle Comstock oven is rated at 25,000 BTU and is a countertop oven. Otis is a good guy, and if you go to the Members section and search under his name, you will find his email address. If you shoot him an email and tell him you are a member of the forum, I am pretty certain he will reply and offer an opinion on the Fibrament stones, if in fact he did replace his damaged stones with the Fibrament stones.
I think that calling Fibrament and speaking to someone there about your replacement questions should also prove useful.