Most (if not all) of the states require a speedo for the bike. So unless you know somebody...wink, wink, nudge, nudge....you bike isn't going to pass (unless the inspector isn't paying attention). You can easily mount a mini-speedo in the back of the headlight an nobody will notice it.
That being said, cben is right, use a washer the same thickness and sealed bearings. Or you could just make a block off plate for the hole (you will still need the spacer). putting a working speedo drive on a cb750 fron end is as much a pain in the ass as deleting it entirely.
Another option you might consider is to use a goldwing front end. 75-77 is a bolt up, you get 38mm forks and an aluminum rim (instead of steel like the cb750) . Plus you get dual discs and a working speedo drive that isn't a pain in the ass and better calipers to boot. The whole thing is a bolt up with the right temkin bearings (I think it is goldwing lower and cb750 upper but I might have this backward).