Size v Power
ABGD The power required for an aircraft has more to do with its weight rather than size. As a 100% replica would be very light compared to the original (for many reasons) 4-500 HP V8 would be adequate and very affordable. Engine/prop packages are already available of the shelf (in the States) and the thrust via the gearing would be excellent.
As the machine does not have to be stressed for combat or carry a war load the construction is more akin to the fast retract singles that Piper and Cessna have produced in the past,but with a slim fuselage and only two seats.
A 180-200 knot cruise and an authentic sound coupled with a lower wing loading should make for a a safe handling machine that looks right,and could acccomodate two full size pilots with ease.
I suspect that the 'scale' issue has more to do with regulations rather than the cost of going the extra mile to full size.After all the cost for these 70-90% versions is quite high for something that does not really replicate the original's classic lines.