There's proably one out there for free, but I couldn't find any. If you join the vintage viking groups.io they have all the manuals. There's lots of help and advice to be found, even expert repair guys are active there.
Cleaning and oiling one of the old machines are a bit more work than they originally were. The main thing is the lubrication free gears, which isnt lubriation free anymore. With regular service they were meant to be oiled with a very stable and slightly thicker oil. The gears are sintered and porous, and you can either start applying a regular machine oil more frequently, or track down a replacement for the original type oil. If you search for it, you will find everything on the group.io. You will get good help here too, but for the manuals and techniqual bit, this is a good supplement.
https://groups.io/g/vikingsewingmachinespre1980/topics