Still not certain it'll be a repair, but either way it starts with taking it apart. The goo was too deep into the machine and internal levers were bent out of shape, so the only way it could be fixed was taking off the typebars and intermediate levers. Did not want to take off the carriage this time, but dismantling the ribbon and spools mechanism and pulling out the ribbon feeding shaft makes way for the fulcrum wires to pass.
These are fairly solid wires that take some tapping from one end to work loose. (The flat head of a small Torx screwdriver helps here.) Tapping from one end will extend the wire just enough to use pliers to pull the fulcrum wires out completely.
Both the typebar and the intermediate lever fulcrum wires are held in place by single locking plates at the sides of the segment. These need to be removed or can be tilted out of the way.
Now then to clean a very gummed up mechanism. On the separate levers different techniques were tried. Mere scrubbing or polishing does not work on this - damages the parts more than that it removes the tar. Tried soaking (days!) in commercial de-greaser to soften - this barely had an impact. Tried soaking (days!) in white spirit (i.e. petrol) to soften - that did indeed soften up the tar, making it possible to rub the parts clean. Also tried soaking in washing-up liquid with just a bit of hot water - that softens up the tar within minutes! Then the parts can be rubbed clean with a rag.
From these experiments, the 50% washing-up liquid and water clearly is the preferred solution. It's effective, fast and not hazardous. Also not risky for any rubber parts and the machine gets a wonderfully fresh lemon scent too!
One of the intermediate levers was bent out of shape. With the part out of the machine it can be gently formed back into shape.
The levers can be soaked in the soapy solution. The whole machine and segment proved harder to get at - putting the roasting pan to good use then also dunked the entire machine in the solution. With vigorous scrubbing then much of the tar was removed. One mistake was to leave the machine in the solution too long (overnight), blackened screws then developed rust.
Luckily many of the internal parts seem to be nickel-plated (these machines must have really gleamed when factory-new), the rust could be mostly wiped off easily and a little oil applied. Now a clean segment again:
Also the typebars are mostly clean.
By the way, the parts that are shaped for their position are very helpfully numbered. All the intermediate bars are identical and not marked. All the keylevers and all typebars have the number of their position in the segment stamped into the part.
Otherwise it'd be a bit of a puzzle. Come to that - it is a puzzle even now :-)
Slow project - it may become a repair job yet!