Hi Harry welcome to the club. You need to think long and hard about this conversion. Many people have tried to achieve that golden chalice of fitting a powerful engine in the back of the diminutive 126 and failed miserably. The FIRE engine conversion has to be the most successful conversion yet with many examples being done (such as the one in your above photo) however you own a BIS which sadly is very different to the aircooled one in the photograph. The tailgate means that the FIRE engine will be to tall to fit into the engine bay which means a great deal more work will be required.
Not to say it can't be done though. Here's a clip of a BIS with a FIRE engine conversion with all the obvious problems of that conversion laid bare to see