Well one thing is sure, it ain't Alex Yoong for sure! 
Problem in rating Bira is he lost his best years to WW2. Then again, had the Nazis not invaded Poland, I doubt he would have got a drive with Merc or Auto Union.
I would suggest Kazuyoshi Hoshino as the Great Lost F1 Hope. Everyone bangs on about Senna's opening lap at Donington, but Hoshino's first-ever Formula 1 Grand Prix lap saw him pass 13 cars.
Satoshi Motoyama has to be worth a shout? Granted, most of his success has been in his homeland - but there's been quite a lot of it.
Daijiro Kato was a GP Motorcycling World Champion, and could have achieved so much more but for his tragic accident.
These are all very good calls! 
B. Bira was a very interesting character with many great accomplishments on and off the track, but also seemingly his own worst enemy with chaos in his private life and shady business dealings that more often didn't work out than did, and caused him to lose vast amounts of money, his own and others'. But he could drive a race car, there's no doubt!
Hoshino is the elder stateman of Japanese motorsport, comparable with Mario Andretti in terms of longevity and success - albeit, Hoshino's success was more local whereas Andretti did it on the grand stage. I believe he holds the record for most Formula Nippon titles, and was also successful in sports- and touring cars.
Motoyama is another highly successful driver on the Japanese national scene who won multiple titles in open wheel and sportscars, and is still going strong today.