Which motor is it?
The Lister-Petters had numerous other uses... truck refrigeration, construction, etc.... and there are likely to be a fair few of them around, here in the UK.
Lister tend to be reasonable for spare parts.
Check out the contacts....
http://www.lister-petter.co.uk/contact.php
I notice that they do not list marine engines at that current website, but they should support your old engine.