Those half round pieces are called woodruff keys.
If you have all of the bolts and screws lose it should slide out. you might have to use a wood dowel to tap it out. Do not tap on it with a steel hammer! It would spread out and get so tight you would not get it out.
Support the parts well when tapping on the shaft because the lifter ams can break.
Tap the shaft with wood dowel in the direction shown while supporting the lifter arm.
Also when you get all those parts off, put them in a container of phosphoric acid overnight.
They should come out looking grey not rusty. Then rinse with water and spray all the parts with WD40.
The reason the shaft does not want to slide out is the corrosion on the shaft that is exposed .
Try to sand those areas so the shaft slides out easier.
Now for some ugly photos.
See all that corrosion on the steel shaft between lifter arms? That is why the shaft will not slide out.
Make sure to treat the shaft with phosphoric acid to treat and stop the corrosion.
Perhaps you can get a friend to help hold the lifter arms while you tap on the shaft with a wood dowel.
On a Roadstar Adventure.