It was common with Hillman Imps, I did this conversion to my Imp. It's a great solution to cooling issues and eliminates the need for power sapping engine driven or electric fans except when in stationary traffic.
I did the job twice, first was a 'quick' attempt using pipes under the car, later I did it properly inside the car, which enabled the use of bigger pipework. I used a mixture of copper pipe (normal plumbing stuff, with solder joints, elbows etc. to take it under the rear seat) and flexible.
I must point out that having a pair of hot water pipes running along the floor did make a noticeable increase to the in car temperature.
I don't think there is much point though unless you get your 126 making more than about 50 horsepower.
