Thanks for the help! That's about what I was guessing. I think the best option is to do whatever I want to it to make it a bike I enjoy riding. And then ride the heck out of it. Right now it is my daily commuter to work. I suppose I just wanted to make sure I wouldn't be breaking anyone's heart by modifying a "rare vintage piece of history" with a few modern components, and foregoing a proper restoration.

Had trouble with pictures before, trying a different computer this time. Oh and I realized I mis-represented the speeds earlier out of ignorance. It is a 6-speed bike, or 6x2. 6 cogs, and 2 chainrings. You get the idea

To answer a few of your great questions:
1. There are no tubing decals left on the frame, just the "Sekai" label on the front. No help, I know.
2. Rear Derailleur is "Integral Mount", bolted directly to the frame.
3. Handlebars and chainrings are steel
4. Seatpost and cranks are aluminum

Still having trouble with the pictures, not sure why it's not letting me attach them, sorry! I'm probably too new to the forum.