Before I finished the hobby, I thought about building an "ultimate" open bike....just for the fun of the challenge. My pushrod Honda V-twins were such a wide engine that I never went onward.
The whole idea was that EVERYTHING has to have air flow across it when you are going fast. Even things that are out of the direct airflow are still creating some air drag that adds to the total.
Those notes on ceramic bearings and such are very good. I did go with those on my bike, and every little thing helps.
One of the ideas I had, which would aid aero without adding out-of-rule items, was to install the battery/box as low as possible just ahead of the rear tire (inline mounted). There is a hard, high pressure area in front of the tire because it drags air around with it....and throws it into the oncoming air ahead of that tire. That concept is why so many modern cars have spats hanging down in front of the tires. Aero drag improvements help the mpg ratings and the manufacturers use those details.
When we prepared the Bonneville Prius (in 2004) with Car & Driver, we were told by Toyota engineers to lower the car until the spats could just touch the ground. They knew very well how much drag a tire can create.
In the bar cables could reduce a little drag, especially it the bars are not round section.
Anything that air flows across should have sharp rear corners (teardrop idea is great, but doesn't really work on a short bike). Look at the new cars and you see more sharp corners at the back. The whole principal of Kamm is a clean break. As proof of little things adding up, look at the top of pickup tailgates, these days. They all have added top width, sharp corner, and tuck-in for better air release of the high pressure stream that dives off the top of the roof. Those folks are good engineers and we should all pay attention to what they are learning. The latest Tacoma has air straighteners to keep airflow under the mirrors from tumbling through the front door handle.
Everything can make a small difference. You could really have fun with your project and perhaps open a new chapter in un-streamlined bikes!
Good luck and success, while you go forward.
JimL