The Detroit’s like almost all industrial, marine, medium duty, and heavy duty engines use SAE pattern bell housing sizes. A quick google search of “SAE bell housing dimensions” should help you sort out what you have.
Like I touched on in your other post, the two stroke Detroit diesels were available in multiple SAE bell housing sizes. The engines were modular in design. The flywheel housing was a separate part from the rest of the block, and I believe it was also interchangeable with the 8v53. Unless the old motor is already gone, swapping that flywheel housing is probably going to be the path of least resistance. The reason they likely went with a smaller bell housing size on the marine engine is there was no clutch, just a drive flange to mate up to the marine transmission.