Sorry to hear you have lost Bailey, I really feel for you. Poor Holly has lost her playmate which is why her personality has changed. I am no expert, but I think the sooner you get another kitty the better for Holly to adjust to a new companion. I am not saying it will be plain sailing, it might take a little while for her to accept the new arrivval. There again she might instantly take to him/her. 4 months ago I introduced our new arrival (Merlin 13 week old male kitten) to Murray (17 year old girl kitty) and Jasper (12 year old male kitty). Jasper took to him instantly and they became playmates, but it took Murray a month to accept him.
Whatever you choose to do good luck and I hope all goes well for you