Jiri at Vintage Replica makes very good battery retainers (but I don't know if he does them with the post-1940 fixed crossbar).
http://www.vintage-replica.cz/svetlaE.htm
No-one really makes an accurate wiring loom, but you have the bike handy so you have a jig ready. There really isn't much to the loom on this type of machine and there are no taped section. You can improvise the thin rubber tube round the headstock with a section of bicycle inner tube.
Replica saddles are generally pretty awful but the later-war 'universal' frames are not that scarce, but you will need to keep your eyes open.
For this era of machine, it's not really a case of a one-stop shop. I certainly wouldn't have started on it if I didn't have an addiction to auto-jumbling.
The alternative is eBay and lots of luck (or lots of money)
Thanks for the link. I sent Jiri a message about getting an un-plated set of straps for the battery holder. I can spot weld on a cross strap but don't have the fabrication skills to make the rolled bend at the bottom.