Thank you everyone for your replies.
I was considering solar as an option. I've got a couple of small panels and control units from a previous project, but there's not enough sunlight at the moment to test everything fully. Another battery would be cheaper than an outboard, but as BilgePump says, there is not a lot of surface area, plus I just can't shake the idea of having a more traditional back up. I'll keep investigating this avenue though.
Attached are pictures of the diesel engine, a Penta MD1. Knackered, in this context, means well beyond my skill to repair and un-economical to pay someone else to do it. I have no idea how much it would cost to get a boat yard to haul the engine out, get it up and running, and put it back in again, but I suspect it would be more than I paid for the boat. Same with having it removed and replaced. I wonder how much I would get for it as scrap?
From a manoeuvrability POV, the electric motor has reverse, so as long as I didn't completely flatten the battery I could use the petrol engine for the long straights, then the electric for docking.
Going through my photo's I have discovered that the outboard bracket is a Seagull. Maybe a clue as to what engine was used in the past.