Seems if your replacing a like engine, most needed material should be available from the old engine, however with DDC, there is no end to the possibilities of what could & should be different, My education started by changing the transmission in the MC7 from the origional 4 speed to a RTO910 Road Ranger, flywheel, bell housing and engine mounts just for a start, DDC / Greyhound used a SAE 1 Bell Housing with a SAE 2 flywheel, that means the starter had to be moved closer to the crankshaft, so a different SAE 1 Bell Housing was required, one for the SAE 1 Flywheel, can get very interesting. Easy if you have a yard full of junk DDC engines.
Good luck
Dave M