Hello Motovita,
Glad you enjoyed the videos.
At least in Spain it is a quite common mod. The airbox comes from a standard Bultaco 183. It was an earlier model but the airbox was much bigger´and carb breathes much better. It fits straight in. If you fit it to a 199 you dont even have to paint it.
Here is a detailed inside picture (found it at todotrial, the trials online reference publication in Spanish):
Regards,
cota200