If your running against 50 hp inteks, that Oppy wont have a chance!
How much HP depends on alot of factors...best I have seen proven was 37 HP.
On the Dual carbs debate....Yes Dual carbs help, alot!
Anyone who says they ran against one with dual carbs and it was not fast...whoever had the dual carb engine screwed something up!
The biggest hold back on an Oppy is not the carb, it is the manifold, mount a pair of carbs on some straight shot manifolds will cure that problem. There is another cure ....but I ain't giving the secret up yet!
Crank and rods will make it a 46, but yes the 46's have some additional clearancing on the block for the extra stroke. Alot of the 42 cube engines already have that done to them...but not all. It is easy to check, there will be some semi0circle cuts at the bottom of the cylinder.