I often disagree with TT's certainty that Honda's design and engineering are perfect but the facts on the ignition system are hard to dispute. The stock Kettering system works fine and no "improvement" to it is going to give you a meaningful horsepower boost.
The original question was about a failing ignition system. Repairing it with stock parts may be an option (are new OEM coil/wire sets still available anywhere?). Using the available new points sets is a dubious choice, many of us have found these to be poorly made and short lived.
Using new five ohm Dyna coils is reasonable. They mount on the stock posts (with slight modification using a file), the low voltage wires plug right in, the HV cables are replaceable and use standard automotive distributor connectors at the coil. You can get new NGK plug caps, either resistor or direct type depending on your thoughts about secondary circuit resistance, if you want a stock look. You can buy neon coloured HV wires with molded on plug caps if you like that look.
There are identically packaged coils (same manufacturers as the failing OEM ones) with removable HV wires. The CB900 has 5 ohm ones I think. The early GL coils fit but the primary resistance is lower and use a ballast resistor much like old cars did, bypassed to give a good strong spark while the starter motor is on (this drops the battery voltage to round 9VDC). I would leave this resistor in the circuit, personally. The coil was designed to give a fine spark with it in at full battery voltage (13-15VDC) so why remove it? The 550 has a tight power budget - the alternator output is just slightly higher than the normal electrical load. Increasing the ignition system power draw by using a lower resistance coil that does not increase performance in any way is not a smart move.
You can use just about any 5 ohm 12V dual-fire coil if you can get it to fit. Don't use extreme low resistance types used with some electronic ignitions, two ohms or lower. You cannot use these "as is" with stock points anyway, and usually they are pulse type coils incompatible with points.
Replacing the points with an electronic ignition is another choice.
The simplest aftermarket electronic ignition unit is the Dyna S, which does work but brings some problems. It draws more power than the points system: the points open (producing a spark) and stay open for just under 1/2 of the rotation of the points cam, giving a fixed ~190 degree dwell angle (if adjusted correctly!) whereas the Dyna circuit -triggered by a magnetic sensor rather than mechanical points - opens (and makes a spark) then closes again in a very short (unchanging with RPM) time. The effective dwell angle thus changes with RPM (total time per shaft turn drops as rpm rises while the "open" time remains fixed) but at normal RPMs the coils are ON more of the time than with points. The Newtronic (and the old Martek) unit has the same features/drawbacks, using an optical sensor. These all use the original mechanical spark advance mechanism with its - adequate - primitive advance curve and inherent instability.
Dwell angle is defined as the fraction of a full 360 degree rotation of the distributor shaft that the points are closed. We don't have distributors, though... anyway it amounts to the fraction of 360 degrees of point cam rotation that the points are closed. The coil requires a certain time (not much time - a few milliseconds) to saturate, meaning to create the maximum magnetic field strength in the coil core. After that time it still draws power, but the power just heats the primary coil wire and does not make more spark output power - the spark energy is derived purely from that magnetic field collapsing when power is removed. The maximum RPM of the engine is partly limited by the coil saturation time and dwell angle, at some very high rpm the points will not be closed long enough to completely form the magnetic field and spark energy will fall proportionally to the reduction in field strength. There is no benefit to a long dwell time. Too short and the spark will get weaker. The stock points system is fine to well over 10,000 RPM.
The Dyna 2000 or Boyer Brandsen micro digital ignitions are very nice - and pretty expensive. They eliminate the mechanical advancer and allow for changing the advance curve. They do not waste much power. Both require special coils, Dyna wants 2.2 or 3 ohm coils while the BB needs a very low resistance pulse coil and -for an SOHC4 - uses four small single coils rather than two dual output coils.
"Hondaman" (here on the boards) is selling an electronic ignition add-on box that uses stock points as a low power switch triggering an electronic coil power switching circuit. This vastly extends the lifespan of the points assembly. The stock advancer is kept, and the dwell angle is adjustable via mechanical adjustment of the points themselves just like with the stock system. Power draw is essentially identical to the stock system. You use stock coils or 3 ohm coils with a ballast resistor. The system is all in a small box that connects between the coils and the wires from the points. Since the points contacts do not wear (the wear is caused by the tiny spark every time they open when connected directly to the coils in the stock system, there's no spark with the Hondaman system) the timing doesn't have to be set very often - checked yearly is about all.
