The twini-mini is not a hard conversion as the gearbox is mounted under the block and the subframes for the twini's you can buy off the shelf.

The entire mini range has been twini at some point including the mini moke
http://www.minimarcos.org.uk/altpics/twini.html

The twini has even been documented in manuals



There's even a twini's about that runs 1480cc metro turbo lumps and goes very well