It's a WWI biplane...with high lift airfoil wings...
You're going to have substantial trim changes with changes in the power setting / speed...

JMO, but, It is what it is...

The stab incidence change helped (possably a lot ) but what I'm getting at is that you're going to have substantial changes in trim / attitude when you make substantial changes in throttle...just the nature of the beast.

You could re-design the whole airplane...with fully symetrical airfoil(s ) and everything 0-0-0 but then it wouldn't be a BUSA Nieuport any more

My friend's had so much down and right thrust that the engine's (G23 ) shaft comming out at such an angle (or compound angle? ) that it just looked goofy...and it still had a strong tendancy to climb with increase in speed...and decend when power was reduced...much like a trainer...he just learned to live with it.

I think I would have shimmed the T.E. of the top wing, and modified the interplane struts.