As far as I know, Husqvarna used sintered gears on all their self oiling modoels, which means porous metal gears treated with a particularly stable and durable oil. It was adviced never to add white spirit, oil or grease to these gears, but as the years go by they need to be retreated with the same type of oil. It's a bit heavier than regular sewing machine oil, and not just any motor oil will do since they tend to be full of additives not ideal for these gears.
What you can do is disregard the original advice, clean best you can and add a bit of white spirit to all gears, to loosen up old oil. There was someone who tracked down a service manual for a Pfaff with this type of gears, and the manual insisted only on adding either HLP 80 or HLP 46 oil. I don't think this exact brand and name exist anymore, but there are equivalents available. We might just be a bit too careful with sintered gears because the sewing machine companies insisted never to oil or grease them ( sometimes with an exlamation mark and warning triangle). Any pure oil about SAE20 is probably right for these gears and any basic oil will be better than completely dry gears. Using a lighter oil can ruin the self oiling properties and you might have to stick to oiling regularly in stead. If you get the right kind of oil it will make the machine self oiling again.
Either way, the reason for a seized machine is often thread in the bobbin area, clean around the hook, race and feed dogs. Thread or pin caught some where is of course a common problem.