My friend got back to me. His comments below...and a few photos. Funky pattern on that seat!
"i did have a look at it and must say that it is an interesting story that goes with this Batavus TS50 Superport. It looks like an original bike with the wrong headlight. Over here in the Netherlands they are not hard to find (headlight i mean).
The engine is a much looked for item but the second gear most of the time is bad. Replacement parts for this second gear are not nearly (hard) to find as are the TS50 JLO engines in general."
He also sent me some screenshots of the engine diagrams that I can send to you if you're interested.