I've reduced vibration in the handlebar by adding end weights, 1lb each end, and new mounting rubber; vacuum sync of the carbs goes a long way too. You still have two pistons moving up and down, and, since it is larger than the 350 twin, has more vibration that just can't be eliminated. The Honda fours were much smoother than any of the parallel twins, almost as smooth as the Goldwing boxer engines.
There is more than one electronic ignition available for the 450, and some include setups for other bikes. Usually, it just involves pulling the points and installing the electronic sensors, and a bit of wiring. If points are properly adjusted, starting the engine is little different compared to electronic. It's the frequent adjustment I wanted to eliminate, since mine has two sets of points to keep in adjustment. Sometimes, I kick it over, sometimes use the button, as the mood strikes. I'm more likely to use the kick when it's warm, because it seldom needs more than one kick.
Swapping brakes from drum to disc on the 450 involves replacing the front forks, brake control lever, and wheel, since the drum brake is part of the wheel hub. This may be true of many others, assuming there are compatible swaps. Not sure I'd bother, as I never had problems stopping my S90 or 305 Super Hawk, both with front drum brakes.