Typically, you use someone's current pronouns, and in cases of celebrity you might include something like "nee Bruce Jenner" in a parenthetical.

"When Jenner won the 1976 men's decathalon, she was ecstatic."

That seems reasonable to me. Pronouns are mostly there to define a referent, not to make you think about someone's gender presentation or genitals every time you see the pronoun. So if you have a long article about Caitlin Jenner and you consistently refer to her as "she," I think it won't actually be jarring. (Similar to how if you repeatedly use "... he said" etc. in writing, it subconsciously disappears to readers, but if you as a writer decide that it's repetitive and you need to use other synonyms for "said" every time, that's actually distracting.)

Of course maybe the real answer is to ask her what she'd prefer?

This reads fine to me? Yeah, Jenner is trans. Yes, she ran in events when she was younger. Is anyone reading that not going to understand?