Barry: Have found a really good site on the Stockholm, now Athena, as below. Engines were replaced in 1989 during one of her many reincarnations, this time as the Italia Prima. She is due back in Dover again on 3rd and 7th September.
Main Engines:
2 x Gotaverken diesel engines, 2-stroke/single acting, 8-cylinders
Total power of 12.000 BHP (8,900 kW)
In 1989 she received - 2 x 8 Cylinder Wartsila Diesels
http://www.ssmaritime.com/Stockholm.htm
There are an excellent series of photos on Dover Ferry Photos Forum and Nigel Thornton (DoverT) is going to compose one of his superbly comprehensive "Past and Present" articles in the near future.
http://www.doverferryphotosforums.co.uk/viewtopic.php?f=14&t=1178