no matter how much you modernize a cb450 setup it is still inadequate for the weight of a 750/550 and you will be operating at that drum's limits pretty much all the time.
GT750 and GT550 suzukis came with massive 4LS front drums which work excellent and since the Waterbuffalo is close to the cb in weight it is a better starting place. A lot of vintage racers use these drums so the price has been driven up in recent years (expect to pay $300-$400 for a good one ready to use) but they are still floating around in yards and occasionally I see GT550s being sold as cheap parts bikes with one.
Other drums to consider are BMW and moto Guzzi drums. Giant things that wil haul most heavy bikes down from speed. Some of the Guzzi drums can be had pretty cheap.
If you really want to spend the money you can still get Grimeca 2LS drums, but expect to spend more than your bike is worth.
Stay away from brit bike drums. Brit bikes drums are awful and the bikes weigh less than a SOHC4.
However, other than posuer coolness there is no advantage to running a drum in this modern era unless your race class requires it. A disc brake offers two advantages over a drum that far and away overpower any need for one:
1) repeatability. Sure that first stop of the drum brake will be better than any disc. And maybe that second will be also. But really once fade sets in the drum is going to be less and less efficient and more unpredictable. With a disc, your 100th lever pull will be fairly close to your first and your 1000th. Every watch the old Car and Track road tests speed network used to show? Those guys would brake test old drum brake cars and by the 10th panic stop the car was already using up an extra 30-50 feet and fade had taken hold. It doesn't take much to use up a drum
2) Weight. A drum is a big chunk of unsprung metal. That is a lot of extra weight you don't need slowing down how you accelerate. Adding unsprung weight to a bike is probably the worst idea possible if performance was your goal. If you can find other places to take the weight off them maybe it won't hurt you too bad but really what are you after - a good looking bike that can't get out of its own way or a good performing bike that is a joy everytime you ride it but doesn't give you the vintage look street cred the other cafe posuers clamor for? If it were me I would rather have a fast bike than one that looked fast but went slow and couldn't stop.
