Thats a bike I like.... No faster than the CB360 (had a HP or 2 less, oddly) but far sexier. I lusted for them when they were first sold.
How did the bolt break? that may define how hard it may be.
Did you try to unscrew it and it snapped, or did something hit it?
The location looks to be in the front of the engine, near the oil filter. In the long run, you may work on it easier with the engine out of frame for access.
Sears (Not sure where you are located) and some hardware supplies, have reverse drills, and ones specifically made to drill into the bolt, grip it and turn it out.
Better than an easy out, as they tend to break.
While you figure out what to do, regularly soak the bolt in penetrating oil. The longer it seeps in, the easier it will be.
It looks to be near the case split line, so at worst, you may be able to split the crankcase and turn it from the inside, maybe continue "tightening" until it goes all the way through.
Heat can help, but keep on the penetrating oil at first, and keep it soaking.
After replacing, use a little never-sieze on the threads, so it doesn't bind up again.
That's as much as I can help, maybe someone here has some additional or better advice....
Hope it gets fixed. I like seeing those baby 4's on the road....