Does anyone know of a compnay that makes a kit to make a convertible (not a roadster)?.

 

There is (was?) a company in the UK that made a kit for the Mini called Keith's Convertibles. They did mine when there and I love it. I believe they are still in business but their website is gone.

Trouble with doing a convertible with the Mini is that being a uni-body, when you chop the roof off you lose a ton of rigidity. Both Lamm and Rover dealt with this by putting added supports in the floor and in the window pillars along with other places I'm sure.