CB400F. They are light and very nimble. You can bore them out to almost 500cc without an issue (and bore the carbs to flow as needed). Lighter set of wheels, a sexier tank (maybe a Benelli Mojave), cafe seat, hidden electrics, single tach with a digital speedo hidden away, modern front end (think upside down anodized gold), and a handmade exhaust. I'd either get new rims to swap in, or a whole new wheel set. All of this can be done surprisingly low cost, and the bike will be STUNNING!!! I just did a similar build on a 350 twin, and I've had loads of people come into my garage exclaiming how beautiful it is.
Camelman