I've repaired many such units in my early servicing days.
Try some Isopropyl (Industrial) Alcohol applied with a stiff brush (a paint brush cut down so that its bristles are no longer flexible but stiff to the touch is usually ideal).
The process could take some time due to the age of the congealed lubricant.
We always used a drop of oil from a can of Shell Handy Oil (now called Shell Household Oil - I think) applied from the end of a small screwdriver. (Squeezing a drop of oil onto a small screwdriver blade and then applying it to the parts requiring lubrication ensures that you do not over-lubricate as this results in oil spraying or dribbling onto drive surfaces).
A long-handled screwdriver will allow you reach "difficult positions".
Isopropyl alcohol is also safe to use on rubber drive (intermediate) wheels and drive belts that require cleaning, although any such rubber parts would (probably) be un-serviceable after 50 years, due to hardening (perishing) of the rubber.
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