My Prima was marinised by a company called "West Country Marine" in Bristol. I don't know if they would sell the parts separately but you could always ask... their number is 0117 960 6611. Alternatively, it would be worth giving Lancing Marine a ring on 01273-411765. I'm not sure if they do parts for the Prima but they do marinising parts for quite a range of engines......
Lancing Marine do a kit for £696.
If you look at their site Http://lancingmarine.com/
You will find it listed as Rover Montego/Sherpa, as this is what the Prima was originally developed for. Before Volvo took it painted it green called it the TMD22 and charged the earth.
Regards,
Brian