She must surely have been a beauty when she was young.
Literal
She [topic-は] young time surely beauty was [must be-に違いない].
さぞかし (surely, no doubt) adds emotional certainty beyond what に違いない alone provides — the speaker can vividly imagine her past beauty. さぞかし is more literary and empathetic than plain きっと (surely), often used when the speaker sympathizes with or marvels at the situation being described.