Shimano Makes 145 tandem hubs .. spare parts are common then. 40 & 48 hole *
perhaps you can re axle a 6 bolt or centerloc Disc Hub with that longer axle ? wont be drilled for so many spokes..

last time I Looked they* were still, threaded on the left end for Arai Drag Brakes ,

but there are screw on to 6 bolt adapters too.. DT for one..