Andrew ,
You had a pretty good list but you left out Velocette motorcycle engines.
Both the LE and Valiant had Boxer engines.   The LE was even a wasserboxer.
200 cc side valve.
In the old country it was often referred to as a Noddy bike as the police
used them a lot.
There is a very nice example of a Valiant in the Transport Museum in
Auckland.
200 cc OHV
There was also a Swiss made boxer twin called a Condor that dates back to
about WW II      It wouldnt surprise me if they were still in use.

Cheers and Happy New Year,
Steve Cotsford