If a device met the code that was in effect at the time it was installed, and if the code changed in such a way that made that device no longer legal to install, that does not mean that it has to be removed or replaced. If the authority having jurisdiction (i.e., the Electrical Inspectors, not a Home Inspector) judges that the present installation represents a hazard to life or property, then they can write it up as a violation of the present code. I don?t think they invoke that right very often, since it is hard to show that a hazard exists.
So the best you may be able to do is to report that this service panel is of a type for which a number of problems have been reported over the years, and recommend that an electrician be brought in to conduct a detailed inspection. Meaning no disrespect to, or unkind reflection upon, yourself or other members of your profession, I think it is too much to ask of a Home Inspector to discern the difference between an old, but still functional panel, and an old panel that is about to fail. You should leave it up to the seller and any prospective buyer to decide who pays for the electrical inspection, and who pays for any repairs or upgrades that may be recommended by the electrician.