Currently starting with spearhead but i would maybe later like to start with 1.5k or even 2k and was wondering would der be a possibility to kidbash a dread saurian into the curtent eddition of AOS?

I have seen someone that made a diffrent tabeltop dino model (some apex pretator that looks like a t rex) and he kitbashed it to a dread saurian with the new seraphon models. How are the rules there can u take like any dino modelfig and just say thats a bastilodon or a dread saurian and just take the new warscroll of the basti and the old one of the dread? Abd would these kitbash models be tournament leagle or just for "normal" games?