Laurent,
I just went through this exercise not 6 months ago...using Solex DDH carbs. I couldn't afford a good original airbox, so I used UNI Filters with angled necks. They easily clear my standard brake booster. Yes, I had to re-jet my carbs, but didn't have to change the chokes. (my carbs are quite different than the PHHs) Re-jetting isn't so bad and Ruddies-Berlin should have all the carb jets and rebuild kits for you.
www.ruddies-berlin.de
Here are the filters I used... I am very happy with them. They fit nicely over the original velocity stacks. The red pre filter slips over the black, base filter...so you can choose what color you like best. The red, pre-filter is only for very dusty conditions.
The clamp-on, pod filters from UNI are Model UP-6245 AST
http://www.unifilter.com/clamp-on/
Also...here's a link with some very good info on tuning the PHH. The photos can be a bit of a pain to download, but makes a good reference.
http://www.toymods.org.au/forums/tech-conversions/19030-how-tune-your-solex-carburetor-images-procedures.html
Good luck finding an original airbox. It can fit on a standard brake booster, but you will need to re-form the bottom of the main air chamber (with a hammer) to clear the taller booster. I may know of someone who has a couple of airboxes... send me an email and I'll send you his info. I have seen a few on eBay, as well.
I'm quite happy with how my vintage carbs turned out, so don't give up too soon. Make sure the throttle shafts are not worn, as you will have a vacuum leak and that will cause poor idling among other nasty issues. The housings can be re-bushed to solve the problem.
Lars is a wealth of good information. I'd pay attention to what he says
Good luck!
Ed Z