Mounting a Hive Crone with Harpy parts
Yes, you can make the head swap. There are 2 rules of thumb for converting Games Workshop models:

Friendly games

Here most things go. If you say 'this model counts as a Hive Crone' before you start playing, then nobody will object.

Tournament games

The rule is WYSIWIG (what you see is what you get). Nobody is going to complain that they can't see your model's mouth weapon. They know what the model represents. I've not had any problems adding extra legs, spikes etc. to my own Tyranids.

In summary, its your model - you can make &amp; paint it how you like.