I would definitely advise caution. If the calcification has made it so her ankle can't flex, I think you'd be asking for lameness problems just out of the simple fact of not being able to stride out properly (over time and being pushed under saddle). And it may be tricky finding a show that won't mark against her heavily, or even disqualify her because with a blemish and a slightly off gait, you'd probably get told she's lame.
She's absolutely BREATHTAKING though. I'm not a huge fan of Standardbreds, and she's just an absolute dreamboat. It's a crying shame about the ankle, but at least she can produce more beauties for you!