To a large extent I think it will depend on the overall tone of your novel and the time period you are writing in. If you can keep the sentiment and tone of the period even throughout, then your modernisation of the speeches shouldn't be too obvious.
I have two books by Margaret George:
The Autobiography of Henry VIII and
Mary Queen of Scotland and the Isles. The books are meant to give the appearance that they were written during the lives of Henry and Mary, and yet they aren't bogged down by heavy Tudor-speak. They are both light, easy reads. You can see excerpts of those books and others on her
website. Unfortunately, I don't remember if she used any famous speeches, or how she incorporated them if she did. It's a while since I read them.
I consider myself a
purist only as far as my favourite time period is concerned. Some modernised language doesn't bother me, as long as authors can keep their anachronisms to a minimum and character behaviour realistic to the times. In an age where the internet makes information so widely available, having a whistling kettle in an 1830's kitchen, or tipping a boy a
dollar for holding a horse in Regency London means the book ends up in the charity bag before I reach the end. If an inexperienced beginner like me can take the time to get it right, I expect published authors to make a similar effort.
