Yeah, it is difficult to say for sure. The only time we have seen Godzilla's past self is the hesei series, and Godzillasaurus had tiny spines going down his back. They were probably more cosmetic than practical. But I find it hard to believe that after being mutated by massive radiation, surviving it, absorbing it, and then becoming dependent (he is lured to it and consumes it in 2000 from what I remember, much like all the other movies) on it, that the spikes would not have mutated to support in the same fashion. It's illogical to me that Godzilla's body would evolve to the radiation but his spikes wouldn't. And even if it didn't heal, which it should (bones are integral to DNA), we would still see the damage happen to the spines throughout the film. Godzilla 2000 is a direct sequel to the original Gojira. He wasn't thrashing tokyo all the years in between and likely wasn't blasting things with nuclear radiation. So I think they would have appeared "healed" in the beginning of the film and then see them deteriorate throughout the movie as he used his ray more and more, almost like a slower, circumstantial fashion similar to the way they dissipated during "meltdown" in Godzilla vs. Destroyah.