(Edited)
Stephan,
The arm is indeed bent, but maybe not so much so as to keep a correct cable from being adjusted to fit, but straightening it would seem a good idea. I don't think it is going to heat and bend like steel, as I believe it is a bronze casting or some other similar alloy. I won't try to be the metalurgist here, because there are some amoung us who do it for a living. On the other hand, SRM offers a very nice stainless-steel replacement.
John O R mentioned that the adjuster might be set "way too far in" , but. You don't mention if the lever is in the rod-contact position, but I supose it is. Assuming it is in the rod-contact position, having a straight arm makes the case worse. It looks to me like you have the cable ends as shown in the first photo below. This is different from my '55, so I don't know why the hole in the gearbox tab is threaded and the cable ferrule is not. Earlier gearboxes had a slot in the top of the tab to allow for the cable to be laid in and a long adjuster threaded through the hole in the tab. Then, additional adjustment at the top was available via the adjuster at the throttle lever (not made part of the cable). Perhaps a cable with an end like that pictured in the second photo, with the nut at the backside of the tab, would serve you better, but I defer to others to say if this is a typical approach for the later models.
I hope this provides some additional interesting info and opinion, but I shant go farther on scant first-hand knowledge of your model year.
Richard L.
Oh, one more thing. In addition to the adjuster screw being adjusted wrong, another possibility is that a previous owner had the clutch push rod lever at the wrong rotation on the operating lever splines.