Great Britain is voting for House of Commons on May 6. A nominee of the U.K. Independence Party died on April 22. See this story. The story reveals that when a candidate listed on a British ballot dies, the election in that district is postponed for three weeks, to give the affected political party time to find a replacement nominee. Thanks to Bill Van Allen for the link.
Again — each candidate/incumbent should have a rank order of replacements.
The filing deadline was April 20. That is one reason such circumstances are so rare, since there are only a couple of weeks before the election.