The normal trick is to loosen the cotter pin nut so the nut surface if flush with the end of the that of the pin and then to hammer the pin out.
In this case age and angle of the pin made that trick very difficult, so I started looking for a cotter pin press. There aren’t many around these days as only bicycle cranks still use cotter pins really, but luckily they are of similar dimensions as these cotter pins.
I ordered a cotter pin press from bikesmithdesign in the US. It’s not a cheap tool at around 60 USD, but it is very solid, has a small form factor and more importantly, fits perfectly for the gear shift lever:
Operation is very simple using a size 19 metric spanner or adjustable spanner. Where some serious hammering did nothing; a few gentle twists on the press undid the pin. It just shows that the right tool for the job is extremely important. I can recommend the tool to all Bella owners that are looking to overhaul their engine if you want to get the cotter pin out quick and easy: