The road in front of my house has cobblestones, quite smooth ones and it's only 100 yards to a more modern street, but it's terrible. Cobblestones wreck bikes, loosens fillings and rattle bones, it hurts. Any surface is more suited to ride a bike on. 50 km's of cobblestones is torture, you could do it on an E-bike or moped with suspension and still end up completely knackered. Paris-Roubaix is about suffering, even if you're fully fit and in good health, going slower will only make the torture last longer. It doesn't seem like the kind of experience you'd want make harder for yourself by trying with any physical issue. That's like running a marathon with little sharp stones in your shoes.