@k_b, thanks a lot! I now have the first 12 Motets ready in the shape, they were printed in “Sämtliche Werke I” (1904). So far I didn’t add the text, because I don’t want to sing the pieces.
The second part of the original publication are 12 duets without text (“Sequuntur Cantiones sine textu”). The link on IMSLP lead’s to the “Bicina”, you posted, but these are from a different publication. Thererfore the link is somehow wrong. I’ve got a copy of “Sämtliche Werke”, so I will type them in myself and I will also look through the Bicinia. At the end I will have a lot of material to choose from …
Meanwhile I also found most of the pieces on CPDL as Sibelius files. I was so focused on the idea, to find a way to transfer the Lilypond files to Dorico, that I forgot to look any further. 
I really love this forum, because there are so many helpful people around here! Thank’s again to everyone, who spent some time to help me.
If anyone is interessted in the result, I could post the file here. ( … when it’s ready …)
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