Using rods through the sides into the firewall is called pinning. It is a very common practice. I commonly use pinning at wing hold down locations. and hi stress points on large heavy models. The most common material I use is bambo skewers I get in the cooking isle. There are two sizes about 1/8 and 1/16. One dollar a bag and very good shear strength (can not be beat for cost vs strength).

Butt joints are not very strong. Even well designed laser cut kits with interlocking tabs can benefit from pinning and using triangle stock.

Don't fixate on the engine and miss the tail strength or landing gear though!

That Nieuport is just a big kite though. The 26 is shy of 3D power for it. You won't have to try hard to fly it scale like haha.
So make it pretty.