The Leyland 1500 diesel was marinised by several companies in the UK and many are still in use today. The main mariniser was Thornycroft which was then part of Leyland but is now owned by an Indian company. They can supply many new parts, I fitted a new freshwater pump on mine and converted the CAV oil filter to spin on last year, with parts from them. They will ship worldwide at sensible prices. Thornycroft Home page
The 1500 is Thornycroft T90
ASAP in the UK also supply many parts at sensible prices Marine, Automotive, and Industrial Parts and Spares - A.S.A.P Supplies
Parts up to complete engines are often available on Ebay UK, and there are a few other companies selling parts both for marine versions and automotive versions. Try googling for BMC 1500 diesel.