It seems having an internal engine on a Vivacity 20 is very unusual. As such a number of people on the excellent Yahoo forum have shown an interest in the layout. I have tried to document in this post.
Positioned beneath the entry hatch the engine does not intrude into the cabin too much but allows sufficient access for maintenance. This is balanced by lead ballast in bows for balance.
All of this is then counterweighted with a load of old lead piping packed in the bows.