It isn't possible to 'un-cancel' a transfer (you'll have to ask someone more knowledgeable in accounting as to why this is the case, I'm sure there's a good reason).

You can, however, duplicate a 'cancelled' transfer. Duplicating will copy the relevant info - customer, products, source document, etc - but will set the initial status to 'draft', letting you mark as todo or validate. The new transfer will behave exactly as the old one did (showing up in the customers Sales Order, deducting stock, etc).

Hope this helps anyone who might have this issue in the future.