Although she has not been at her best this year, imagine that she would win this match against the reigning champion, no one would doubt her calibre as a sportsperson.

The original "imagine that she would win" awkwardly mixes imperative with conditional. The correct hypothetical/conditional structure uses inverted subjunctive: "*Should she win* this match…" = "If she should win this match…"

Why other options fail:

• "imagine that she is winning this match against" — present continuous (she's currently winning) conflicts with the future-conditional intent.
• "should she be winning this match against" — wrong aspect; the construction is "should she win" (simple), not "should she be winning."
• "imagine if she win this match against" — "win" should be "wins" (subject-verb agreement) and "imagine if" is colloquial.

Option 1: should she win this match against.