Currently on Piston Heads there is a Mini for sale with a Fireblade engine fitted in the boot. How is this conversion generally achieved, would it be like the Radicals, solid rear drive shaft chain driven or maybe specially made diff?.

Search Google for Z cars and bike engined mini's. Should answer all your questions.

They are typically mounted to a complete cage/sub-frame and bolted and welded into the rear of the mini. Chain driven is to my knowledge the preferred method!

http://www.zcars.org.uk/minikit.php