By the late 20s it was an outdated design with its seperate cylinders. It was basically an enlarged WWI RR Eagle and Curtiss had already shown the way to build a modern engine with monoblock cylinder castings. I think the Buzzard might have been a good engine for boats and a 6 cylinder version (one bank of cylinders) with 300hp could have been a great engine for early war tanks.
It would have been an improvement over the smaller Nuffield Liberty (also a WWI design based on the Mercedes D.III and ultimately the Austro-Daimler 6), but yes, not as much as more modern engines. Though it's larger size, similar to the Buzzard that replaced it, would allow it to be converted to diesel without losing too much power to run a tank. It would be larger, necessitating larger engine compartments (and probably larger tanks), and transmission design would have to improve to handle its power. Those considerations also apply to the Buzzard. As a marine engine, either engine would work well, similar to the DB 602 or the Packard 1A-2500, both aircraft engine designs.
Soviet V-2 diesels started out as Mikulin AM-34, that were inspired by the BMW VI that they were building under license, and first used in gasoline version as the M-17 in BT tanks
The V-2 started out as an aircraft diesel from Charomskiy known as the AD-2. It has nothing in common (to my knowledge) with the AM-34, but might be closely related to the larger ACh-30 and M-40 aircraft diesels, also from Charomskiy. That being said, the AM-34 was used in the SMK and T-100 prototypes as the GAM-34, and the ACh-30 was developed into the TD-30 and M-50T tank engines used on the IS-7 prototypes.
I would love to link the history of the V-2 but unfortunately my main source for this, kampfpanzer.de, is no longer online and the Internet Archive doesn't archive complex interactive sites like that well, so that information is lost.