I don't know about the musician specifically, but at this point, I wouldn't be too worried about preamps. They may well burn out or what have you, but there are a huge number of easy to install replacements that should fit about anything. It won't be stock, but you won't find an original, and it would be less of a sin than stripping and routing a 62 Jazz or something. Heck, it might even be better. Again, I don't know the early Ibanez pres, but a lot of early active systems were not that great in the first place. The bigger concern I would have would be the pickups. If they work, they work (and it is nothing for a pickup to be fine for 60 years or more if it was decently made in the first place and not abused). The issue would be, if a pickup were to fail, I'm not sure that the musician pickups are any sort of standard size, and you won't find an original, so you would have some challenges (pay to get it rewound, get something custom made, use something that doesn't fit right).