Finally, my wooden handlebars came in from Italy. They were made to match the rims. I had to shorten the stem and bolt to get the steering height as low a possible.
The bolt had to be rethreaded but that failed. The bolt wasn’t thick enough and the new thread broke when tightening down the handlebar.
So I made a new bolt myself using M8 wire thread and an m8 nut. My welding is getting a little bit better. 
Handlebars mounted.