Yes... I did not mean to imply they were the same block. The question is did westerbeke use perkins blocks, or just whatever they found that suited the application they were looking for.
Perkins was the first builder of high speed small dispacement diesel engines starting around 1935. So they would have been a great source for small diesels before yanmar and Kubota took over. The MG diesels are not well known to us here in the states, but they list a model b series that was built from 1954-1980 in various sizes and in a diesel version. My guess is the different displacement models within that line could have supported the W30,W40,and W50. W-50 being 1.8 liters at 110ci.
I like finding this info as it does help locate parts much cheaper sometimes.
https://en.m.wikipedia.org/wiki/BMC_B-Series_engine