How detailed do you plan to get with this project? I think you have the basic proportions right, although it might be helpful to see a straight-on side view to be sure. The devil is in the details, IMHO, especially with something as essentially mechanical as a motorcycle. I think you need to look at reference images, see how the engine relates to the transmission, how the carburetors mount, how the exhaust comes out of the cylinder head rather than the crankcase. How the wheels mount to the frame and forks, how the chain or belt routes from the engine to the rear wheel. Find pictures of bare frames to see how they lay out - most (in my experience) have a single tube back from the steering head to a spot just behind the tank where it splits to go to the back wheel. The tubes don’t run thru the tank, rather the tank sits over the top tube and drapes down either side. That said, it’s a good start and I’ll enjoy watching your progress.