Trump will not be able to get a replacement nominated and confirmed before the election. Even an expedited process would take too long.
What this does mean is that the Supreme Court become (even more of) a political issue in play at the election; Trump can use the possibility of losing the opportunity to replace Ginsburg as a means of galvanising Republican voters to turn out and vote.
In terms of whether it would be the end of Roe v Wade if Ginsburg were to be replaced with a Republican - probably not. For one thing, the Supreme Court is already majority Republican, so it isn’t Ginsburg who held the line for democrats. It would obviously be very concerning from a left wing point of view to have her replaced by a Republican - particularly one chosen by Trump. But it’s unlikely to be the tipping point for Roe v Wade.
But even more importantly, the Supreme Court doesn’t actually strictly split down Republican / Democrat lines. The more relevant split is Institutionalist / Radical. Those justices who are institutionalists will almost always vote to uphold decisions which have gone before, and will rely heavily on precedent. The current head of the Supreme Court, John G Roberts is a Republican and an institutionalist, which means he would be most likely to uphold Roe v Wade on the basis of it being settled precedent. It is therefore not at all guaranteed, or even likely, that a majority Republican SC would overturn Roe v Wade.
Interestingly, that’s why you sometimes see Sonia Sotomayor voting in favour of upholding precedent even when it doesn’t seem consistent with her political stance; she recognises the value in the Supreme Court upholding the decisions of more liberal benches of the past, and so she is a staunch institutionalist in order to strengthen that position in the hope that her Republican colleagues will do the same.