It will be a Rootes Group 1600 or 1725cc, same basic unit as went in the Hillman Minx / Singer Vogue / Sunbeam Rapier - and I think the Rapier overdrive gearbox may fit as well, not sure.
Worth noting that the Rapier engine, with its twin carbs, alloy head and different cam, was quite a bit more powerful than the single carb, iron head engines.
As these engines were used as the basis of the engines for the later 'Arrow' Minxes there is a possiblility that the Holbay H120 (120bhp) engine might be persuaded to fit, although finding room for the two, twin-choke Webers might be a problem.