The coils are designed and made by man, so they can certainly fail, particularly when abuse (by man) is involved. But, nothing lives forever, not even the sun. The output lead insulation can get hard with age and lose flexibility. This is not so much of an issue while installed, but moving the leads during maintenance can cause the insulation to crack, leading to high voltage leakage and ignition problems.
Shipping aged coils with hardened lead insulation, can lead to cracks, too. So, consider that when buying used coils.
If the leads crack AT the coil junction, the assembly is quite compromised. If there is enough good lead, NGK makes splice unions to graft new flexible leads onto the old stubs.
If there are hatchet marks in the coils, they will need to be renewed. For the 550, just about any 5 ohm coil primary, dual output coil will "work", electrically speaking. The stock ones are optimized for the needs of this machine. There are those that will tell you it needs an "upgrade". But, most are just repeating folklore without a real understanding of how the ignition system works or what the functional needs really are. I have several sets that prove they work fine for more than 30 years. I also have one coil where the output lead insulation cracked at the coil, (most likely my bad) rendering that coil a "bad" assembly. I'd wager the coil itself is fine. One day, I'll splice new, steel strand core wire onto it and put it back into service. However, steel strand core, 7mm wire is not easy to get, unless I buy a whole spool. $$$
Cheers,
