Bryan's Fan alerted me to a recent eBay advert for detailed construction plans of two classic German wall machines.
There's an associated website which I've added to Archive/Services: http://www.filoumatix.de/
Via Google Translate
The machines are the J&M Bajazzo (Clown) and Jemoc.
Putting aside the challenge and satisfaction of building a Clown from scratch, it's not by any means rare, so obtaining an original may be costly but not difficult. Jemoc, on the other hand, I've never seen in the UK. It's a coin rolling and balancing game, and would have had to be extensively redesigned to work on the old penny (perhaps less of a problem, if building from scratch?)
I would certainly buy plans for the magnificent Zeppelin
Also supplied are some reproduction brass pressings (surround for playfield ball holes), cast payout cups and coin acceptors (presumably for 10 Pfennig).
Here's two German language documentaries about Jentzsch & Meerz and Holger Oelschläger reproducing some of their machines: