Foley beat his opponent more than 2-1 the last time. In 1998 he ran unopposed. In other words, it should be a safe Republican district. If Foley had resigned sooner, another Republican might have won the seat in November. I don't know what the rules are about write-in campaigns, but this close to the general election it would probably require someone who is already well-known to step forward as a candidate.
Too late for a write in.