Your best bet is to look into what the most common nitro variants are and try to use existing designs for inspiration. As far as software for modeling an entire assembly SolidWorks is the best option, Autodesk Inventor, ProE, and Catia are also good but none of them are free. As far as I know there aren’t many free CAD platforms that have support for assemblies. Autodesk does offer software for free to students. You can then export neutral CAD files (.stp) to share and allow people to modify to their needs and .stl to print obviously.