I have those rims. A 406 20" with a tyre up to 1.35 will work fine. I use a 1.1 Schwalbe and I have about 1cm clearance. The bike rides better than on 18x1.5 rims/tyres, bit it is a harsher ride. I use the matching Lite-Pro brake levers, so they adjust nicely to 20" from 18" too. You can get them cheaply on Aliexpress or a similar site. Sorry not sure I can post photos but there s one on the Facebook Global Birdy Forum,.
Thanks very much for the info. I have been tracking the FB global Birdy community and did see someone with 406 rims and Schwalbe tires that had an aggressive tread similar to Black Jacks. I have disc brakes on my bike, so the conversion won't affect the brakes as much.