I've produce webinars for myself and clients, as well as managed a top speaker. The fee is actually based on your current 1 hour presentation fee. Speaker fees for webinar are typically less in-person because you don't have to travel. For example, the speaker fee was 5k, I would discount 20-30% and let them counter back.
Don't recreate the wheel if content is based on an in-person session. You should plan on speaking for 40-50 mins and allow time for Q&A.
Hope this is helpful. If you want to chat offline, send me an email.