If he was in his prime today, he'd be using what all his olleagues at Magnum are using, a Canon 5D.
For his wedding work, he'd be using a Canon 5D.
Although he was never adept at processing and printing, in a modern photojournalist role he'd have to cope with the basics of Lightroom to process and upload his jpegs. If he didn't, his work would be unemployable and he'd not rise above the mass of capable guys out there.
If he was starting today, he'd know nothing other than digital and probably shoot a compact.
He was a man of his time. He wasn't a Leica fan boy that he's made out to be.
I chatted to Chris Steele Perkins in the London Magnum gallery last year and he, like Martin Parr and Sebastian Salgado, uses a Canon. Chris and Marin use 5Ds and Chris explained he did so because it does the job for very little expense. Film in his line of work was a ridiculous idea, he'd not used it since 2004.
He went on to say that he could even remember how to use a rangefinder camera and had no reason to contemplate it when AF is perfect for the work.
Now I'm aware that there are some Magnum photographers that still use film cameras for personal projects because they like film, but even Elliott Erwitt uses digital for his work assignments. Many of us enjoy film, including me, but from what I know HCB was not a hobbyist, he was a jobbing photographer with a great eye.