You can find a solution here:
Frage zu VSTs aus 11 in 12 und zur Installation - Auf Deutsch / Cubase - Steinberg Forums
You can get Loopmash running under C12 by copying the following files or folders from the C11 program folder (or previous versions) to the same locations in the C12 program folder:
“C:\Program Files\Steinberg\Cubase 11\VST3\LoopMash_x64.vst3”
“C:\Program Files\Steinberg\Cubase 11\pulseOSC_x64.dll”
“C:\Program Files\Steinberg\Cubase 11\VST XMLs\Steinberg Media Technologies\LoopMash”
However, it was not possible for me to access the factory presets. They’re in a vstsound file that was probably changed in C12 and shouldn’t be overwritten.
I was able to load user presets.