Lifting 5701Lb (inclusive of two crew and fuel) with only 360HP at 50% scale would seem to suggest 600HP will be suitable for 60% (on a crude equivalence of size between the two types), and composites might provide more strength at a lower weight than timber?, (although the link above suggests the S-31 had a metal monoque fuselage rather than being all wood?)

I wonder if drawings of the S31 survive from Shorts somewhere?, maybe someone can do a 60% Stirling as well?

The S.31 was all wooden and designed to test the aerodynamics of the Stirling at half scale. Shorts had become somewhat adept at trialing big aircraft with half scale models, particularly using a Short Scion fitted with a half scale flying boat hull to test hydrodynamics as well, getting more accurate results than with tank test models.

I’ll leave it to the aerodynamisists to explain how to scale an aircraft down to still give the same aerodynamic performance. The Pobjoys on the S.31 were initially 90hp and later 115hp (Niagara IVs).

I very much doubt any drawings survive but those Rotec radials would be an ideal start point for a recreation. Good luck with the Halifax.