The trigger packs are completely different. No parts IIRC interchange here. You can swap the entire HK gripframe in place of a CETME, but the guts do not interchange.
The buttstock is not interchangeable. They can be modified to fit.
Rear sights on the CETME are more conventional than the HK's drum sight setup. I don't believe they can be interchanged without also changing the welded on sight base on the receiver.
IIRC, the CETMe has two bolt locking levers instead of the HK's single.
I believe forearms and pistol grips are about the only parts that take little/no modification to swap.
While they operate on the same priciple and were generally developed by the same people, their lineage forked far enough back that they are quite different in detail.
