You can contact these people:
G&K Automotive Conversion, Inc.. They can tell you what is legally possible and the costs involved.
Depending on year (Gen 1 or Gen 2) and model (CityCoupe, pure, passion, pulse, Brabus, etc.) there may be different cost factors and valuations. A Gen 2, 84ps turbo pulse or a Brabus will have increased value here. Any diesel may not be possible at all.