The readily available documents are sparse. Allegedly, there was a newspaper account of Murai's encounter with Takeda, but the main source is a rather expensive book by Okuyama Ryuho of Hakko-ryu (Murai oddly became a teacher of this Daito-ryu offshoot after WWII). Surely, Okuyama had his own axe to grind - but maybe it's all true.
There are no additional readily available sources in Japanese on the internet - just a repetition of the same accounts, with no further detail. There is little information on Murai Kenpachi period.
As for Mifune and Murai, what does this mean, really? A much bigger, highly skilled judoka beat a smaller, older highly skilled judoka in randori?
A friend of mine is Japan is checking out if there is any more information on Murai - he sounds like a fascinating individual.
As always with the Japanese martial artists...
Or was created to sound like a fascinating person. Who knows by this point.