[MlMt] How do I change the shortcuts for Message Body Parts?

[Muster Hans]

The default shortcuts for View -> Message Body Parts are alt-cmd-] and 
alt-cmd-[

I am using a Swiss keyboard, which uses alt-5 and alt-6 to represent [ 
and ] respectively.  Therefore alt + [ or alt + ] are unavailable to me.

1. How do I change the shortcuts for these actions?
2. Does anyone have a suitable suggestion for replacement shorcuts? 
(less important)