Thanks all.
I'm guessing that the amount of bonding surface I have is OK since that is how it was supplied.
My easiest glue options (i.e. what I can get simply) are West System 105 or Techniglue CA. Obviously I'd need to thicken up the West System, but with both I'd have added chopped strands of fibreglass to thicken, strengthen, and also help with the galvanic problem. I do have some very light glass cloth, but not sure if there is room for that unless I am very careful or sand things back a bit.
I might also quickly investigate what other products I can get. The old discussions I was searching for did mention the strengths of various glue systems.
I can clean and prepare everything OK, but just had a feeling I should provide a bit more of a mechanical key as well. My thought was to use a tap to cut some small ridges on the alloy part (circular), possibly a diamond cut. Needed? A mistake?