Why would it be illegal? A cease and desist is just a fancy letter saying do what we want or else we’ll go crying to a judge. If the original dev hasn’t done anything wrong (I’m not familiar with the project but I’d still say that’s highly likely to be the case) then there’s nothing to worry about. So make all the clones you want, and watch a dumbass company Streisand itself
They stole from you first, and the bible don’t say nothin about stealing it back