Advice needed--reaching out to a potential advisor I have applied to a Canadian university for a math PhD starting this fall. I was very keen on working with Prof. A and he in fact, had encouraged me to apply. Few days earlier, I reached out to him for an update on my application status and he replied saying sorry that his funds for this fall are already committed. However, he finished by saying that I may find another advisor to begin my graduate studies.

Now I know this does not mean that I am admitted. But can I reach out to other potential advisors now? As a matter of fact, I have been independently reading some new topics and been finding them much intriguing. Now there is a Prof. B who has similar interests and I like his profile too. Will it be worthwhile to write to him now? If yes, how should I broach this subject? Should I mention Prof. A? I don't want to look pushy, but at this time now, I really want an opportunity to start my graduate studies and Prof. B's interests are very similar to what I've been reading recently.

I am looking forward to your suggestions. Thanks in advance!
Of course you should contact him.  There is no harm in that.  But I wouldn’t mention professor A.  Write a simple and short email stating the you have already applied in the department and you are looking for a supervisor for your PhD. Then also mention that you have been reading papers on topic X and you find them interesting. And since his research interests are similar to what you want to do, would he be interested in serving as your supervisor?  And then end your email by asking him to review your application and contact you in case of any question or query.

Good luck!