So, Kentish Town station is closed again due to faulty escalators.

BBC London Travel News stated "Tickets are being accepted on local buses".

How is this handled for anyone who has Oyster pre-pay, and doesn't have a
"ticket"? Will they be charged for an "extra" journey by bus?

Email or call (I prefer email) the Oyster helpdesk and explain the extra cost you
have incurred and they will refund you.

If you will definitely pass through the gates of a specific tube station in the next
7 days tell them this and they will send the refund electronically to the gate for
your card to pick up, otherwise you have to cash in a cheque or voucher.