Hi
If the material the headlight shell, side covers and airbox are made of polyurethane, it's not the PUR i am familiar with. That being said, i am no plastics expert, but this material Honda used seems more brittle than flexible and cracks fairly easily when used. I am not aware PUR has these characteristics and the PUR i am familiar with does not degrade with age like the material used by Honda.
I agree w/ Steve F on saving the airbox, best suited a good shelf item or fit for a museum bike. Getting that restored orginal airbox assembled and mounted in place without breaking it in the process is entirely possible, but the stress of being mounted, heat and gasoline vapors will likely make it crack in time. Wes Anderson told the story of just bringing his long stored sandcast machine into the warm daylight sun, came back and there were large cracks in the box.
I am not aware the "nasty, rotten, brittle" material has been identified. A very reliable, knowledgeable body man i know, was unable to say what the material is.
NOS sidecovers are *not* impossible to find in the desired color, "only" about $500 for a set and a NOS early airbox will be a long time a findin' - likely never. Even if one decides to buy a set of NOS covers, then are you going to be able to live with the ink stamp date ? e.g., 548 likley built April 69. Pretty certain bet NOS side covers will have an ink stamp date well later into 1969, even 1970. I have heard details like this drive purists mad..........
So, therefore, to my way of thinking, one is pretty much relegated to using repro body parts, unless one is lucky enough to have a bike with solid original body parts -or- has access to a cache' of NOS OEM parts that have the appropriate ink date stamp to month/year of a particular machine's manufacture.........
The material the Japanese used is not fibre glass. Nor are there markings on CB750's such as used by inspectors on automobile assembly lines. About the only 'inspection' marks on a CB750 that i am aware of are the ink date stampings on body parts *and* on inside of engine covers and cases. The other assembly completion marking i am aware of are the yellow paint dobs on certain nut and bolts. Further, i am not certain Honda was dabbing yellow paint on the sandcast machines, it is possible this yellow paint dobbing came along after sandcast production era. I am not aware of anyone who has the exact font to replicate the font of the stamp used on these sidecovers.
Seems, with this sandcast restoration 'business', there are always compromises to be made in the consdiderations to replicate "original."