The way I found out my Westerbeke W-52 was a Perkins 4-135 and was also a Mazda R2 block as found in thousands of Mazda B2200 and Ford Ranger pickups was by getting diagrams of the Mazda block and photos of all three from the internet.
See here: Westerbeke or Yanmar??
Go to a Westerbeke owners' forum and use Google. This took me some time, but knowing I can repower by rebuilding an extremely common pickup diesel changed my entire perspective about owning a "vintage" engine.
You can also pay to join Boatdiesel.com. The truth is out there, apparently: