Sure, everything has to be matched together to work. I'm sure designers at the time were well aware of that.
As for a diesel Churchill, the twin 6-71 used in the Sherman produced around 400 hp, which is quite similar to the 350 hp the historical Churchill engine had. So that might have been an option (though the twin 6-71 was a bit of a rush job).
In general, I think not vigorously going for diesel powered tanks (and other AFV's and why not trucks as well) in the pre-war era was a mistake almost every one of the major combatants did. Except for the Russians.
Trouble was that you needed a different logistics train.
Germans used, at time, captured civilian motor fuel in France. Poland didn't have enough to say so.
Of course with MK I and MK II tanks you could top up a number of tanks from one filling station, MK IVs not so many
The US Marines got most of the Diesel powered Shermans that the US kept. They foisted the rest onto the British and the Soviets. US Marines and Navy already had quantities of diesel for landing craft and aux generators on ships.
There just weren't that many Diesel engines available in the late 1930s to chose from for either tanks or trucks. And Diesels were heavy. A 6-71 went about 2200lbs. You needed two replace the radial engine in the early M-4 tank. You also needed to extend the hull about 1 ft in length to fit them in.
In the US there were around 480 different models of Stock, Industrial, Marine and commercial vehicle/automotive engines being cataloged in 1940. does not include aircraft.
from 6hp to over 600hp. Darn few were suitable to tanks.
It took quite a while in the 1950s for the US to switch to diesel fuel for transport. Trying to switch to diesels in WW II ???
The US could use or adapt a number of existing engines in short order for special uses. Waukesha for example supplied thousands of large (like 779 cu in) six cylinder engines for large trucks and tractors either using exiting models or slightly modified ones.
Tanks also needed room around the engines to work on them, they hadn't figured out how to pull engines for routine maintenance
There was a lot more to consider than just getting an engine had the required power.
These used a 210hp Waukesha engine, 6 cylinders, 779 cu in. 1800lbs and the basic engine was in production 1940. These industrial engines were sometimes rated in different ways.
In the High speed tractor they were rated at 210hp 2100rpm. but the basic engine was rated at 168hp at 1800rpm. They were sometimes rated at full load, intermittent load (90%) and continuous load (80%) depending on manufacturer.
This is just an example.
The number of motor vehicles that the US used was staggering. And the US did a much better job of standardizing things than most other armies. A least those that made more than 2 types of trucks (Soviets).
Diesels in WW II were example of the "The Best is the enemy of good".
The Soviets had 1 diesel that they used in the bigger tanks.
14,000 of these used a pair of car engines running on gasoline, as did the vast majority of wheeled Soviet vehicles.
So they needed both diesel tank trucks and gasoline tank trucks to supply armored units.