The purist in me, and others, would suggest you follow your first thought here . . . uh, the 1.2S with twin single carbs and would be by far the easiest to do . . . practically nothing to fabricate and a nice power boost over
the "A's" 1.0 single carb engine.
Any other engine choices require extensive rework and fabrication due to engine size and power (torque) production, not to mention major sheetmetal work in resizing the transmission tunnel, the "A's" is tiny!! Brakes will also be a major problem, drums all around.
That said, not that it can't be done, just be sure you completely understand just how much work will be involved! I'd hate to see this very nice example end up as another "unfinished project!"