It is mine. I painted it in around 1955. It is a portrait of The Queen.
It was never hung but eventually stored in our attic.

At the time I was painting quite a bit but could not afford a decent
surface. I recall that this was on a piece of corrugated cardboard from
the side of a Players' cigarette packing case. I used Dulux undercoat to
surface it I recall.

I think that I gravitated towards egg tempera because it meant that the
brushes could be cleaned with water.

I would now like to restore it and give it to one of my patriotic
daughters now living in abroad. I think she would be the one that would
most appreciate it.

So I retrieved it only to find that it had warped and that one 'streak'
of the paint had cracked revealing a bit of a mess. The dark background
has also faded on one side.

I plan to glue the whole thing onto plywood to keep it flat and then to
restore the paintwork. The problem is that egg tempera is not stocked
anywhere nearby but most shops sell acrylics.

Could I use acrylic for this?

Sorry to be so long-winded.