Dunno 'bout no Hattori Hanzo but the closest to Masamune-san living today is Keijiro Doi. He made his knives (principally sushi knives) in the same manner as second-generation samurai swords, a low-carbon spine welded in a hammer forge to a high-carbon edge.
Unfortunately, Keijiro-san retired in 2011 at the age of 86. Fortunately, his son Itsuo is following in his father's footsteps, and should be as equally skilled as Papa-san.
In another 20 years.
In the meanwhile, Keijiro-san ramped up production in preparation for his retirement and there still are some of his originals available, but be prepared to part with at least 350 skins for one. Most are >$500. If that's too dear, Sakai Takayuki is a respectable fall-back.