Martin,
there are CMK resin engines around for that kit, but a fellow modeller who is building the Hippo Gotha told me that they were awfull. He does a lot of scratchbuilding just on the engines and uses only minor parts from the CMK sets.
If there's no PART set available, maybe you can use some generic PE sets for beezels, MGs and so on? If you like, I'll have a look at my stash, maybe there's a PART PE set for the Roden Gotha in 1/72 left. I could scan it for you and when you upsize it to 1/48 it might be of any use for scratchbuilding parts? Just let me know!
Best wishes
Borsos