If the missing MOK key is the only issue, then you should be able to boot a different kernel.
At the “grub” menu, select the “Advanced options” entry. Then you can select the kernel to use for this boot.
Just in case somebody googles into this at some point in time later. 
This Suse page indicates that the steps listed by Sauerland 
are correct and the way to go - but having followed those steps, i experienced that all options for booting ended up in an endless Boot Screen and did NOT launch (see chapter installation). 
When me trying to upgrade from Leap 15.5 to Tumbleweed failed 
due a SHIM issue, i was immediately able to launch other snapshots and the re-‘upgrade’ back to the original via my Leap 15.5 USB Installation stick worked well - no data lost. 
But with this upgrade of a Kernel, no boot whatsoever worked anymore. 
Even the re-upgade via my Leap 15.5 USB Installation stick gave me a heart attack when presenting a MOK Management screen after installation. 
First i thought that a key needs to be installed as discussed here in this chat, so i tried each of the some key-looking lines in the key menu only for being informed for each that a .cert is missing. Checking the option ‘Enable from Disc’ did provide me wild and cryptic list with lines like $Action and $Boot, non allowed any key selection. 
Eventually i decided to use the ‘Delete Key’ option - which actually worked and all could be launched fine again. 
Having made these interesting experiences with MOK Management gives me a bit weak knees thinking about upcoming upgrades to Leap 15.6 or later migrations to Slowroll. 
Anyway, the help having received here in this forum is fantastic - extremely useful for pushing my process of thinking into the right direction, which is very much appreciated. A big thanks to all of you. 
And for providing at least something constructive:
- Adding the Kernel repository in YaST did not work, but this command:
sudo zypper ar -f http://download.opensuse.org/repositories/Kernel:/stable:/Backport/standard/ Kernel_Backport - YaST did not trigger any Kernel upgrade but this command:
sudo zypper dup --from http://download.opensuse.org/repositories/Kernel:/stable:/Backport/standard/ --allow-vendor-change
It works, but zypper Yast do not switch to the packages of an other Repo by default.
That has to be done manually by f. e. using the second command: zypper dup --from xxxxxxx
After that, zypper/Yast will use the packages of that Repo and also update the packages if newer packages are available from that Repo.