After accurizing the kit to 17.C.1 standards, make the
cowling a horseshoe (basically, the same as a Nie11), add an 80 hp
Le Rhone and you have a 21. Viola! Even easier is to buy the
replacement cowling for the Nieuport 11/21 from Rosemont and
follow the included instructions.
Nieuport 23
This is the easiest derivative to make from the 17.
Perform all steps to correct the basic Nieuport 17 kit. After
those modifications are made make the Nieuport 23 by slantting the
centrally mounted Vickers gun to starboard. Note, however, there
was also a difference in engines used. If modeling a French
Nieuport 23, then use a 120 hp Le Rhone 9Jb. If it's British,
then stick with the 110 hp engine. Again, it is imperative to
study all sources for the particular machine being modeled.