"Would be" is absolutely correct. You could even say "Today we are remembering Jane on her 80th birthday" because IT IS still her birthday whether she is alive or not! However, since Jane is no longer with us, she is not celebrating her 80th birthday. What's happening here is that the speaker is talking as if her ghost were present in the room with us and Jane could hear us: "Dear Jane, this would have been your 80th birthday". When talking to Jane, it makes less sense to say "this would be your 80th birthday" as if she were a living person. Even though she is present with us, she is also "living" in the past so for it to make sense to her you need to say "it would have been". I hope this makes sense to you. The reason we talk this way is to show respect to Jane. She is with us right now, at least in our memories.

I’ll vote with Jonathan on this one ; ‘would be’ is covered by its use as an implied conditional : this would be Jane’s 80th birthday if she was still alive. If we are describing a past event, the consequences of which are also in the past, only ‘. . would have been . .’ is appropriate : ‘It would have been better if the government had not written the guidelines in to law’. The event is past and can’t be changed, and the outcome is also in the past, and no longer a matter of concern. If the outcome is still relevant now, then you could use ‘. . would be . .’ : ‘It would be better [now] if the government had not written the guidelines in to law‘.

The sentence is correct. You cannot change it to “would be”. The way it is written indicates that Jane is deceased.