40–49cm Dolls 40cm childlike female

While not exactly illegal, it is usually considered bad form to try to intentionally sculpt something straight from something someone else have done.
But to simply be influenced by something you like and to try to achieve something similar in your own way is a different thing.
I would suggest that you try to pinpoint what it is that you are attracted to in the body in question. Is it the childlikeness? Do your own childlike body. Is it the proportions? Do your own body with similar proportions (but your own shapes and details).

Another option would be to buy the body and either mod the parts you don’t like (people do that all the time in the BJD hobby) or replace them with your own original parts.

 

Just to do it for your own collection, and making your own from scratch, just using the proportions as a guide but sculpting every single thing yourself? Meh, go for it - it would probably still be okay even if you weren't doing it just for your own collection, as long as you were careful to do your own joint designs and stuff as well, but the doll community can be pretty loud when they think an artist has been wronged (even if it's just "I like those general proportions and want to do something similar") so... just make sure that you use the original doll as a starting point, an idea, inspiration, etc., and don't try to actually copy anything from it, and I think you'll be fine.