On second thought, the Napier Lion will probably need some additional modifications to work well in a tank.
This source states that it was tested for the Cruiser Mk III, and the engine was prone to severe detonation when running at full throttle at low speeds with Grade III 65 octane fuel. This was despite lowering its compression ratio to be less than that of its 2 competitors (the Liberty L-12 and Thornycroft RY/12), which did not have this problem. The other engines had both smaller and larger bores, and both smaller and larger displacements than the modified Lion, so that shouldn't have been the cause of its problems. Both engines had 2 valves per cylinder compared to the Napier Lion's 4, but this doesn't seem to have much to do with detonation or compression ratio, so it probably wasn't the cause of detonation. This leaves just the stroke- 7 inches for the other engines and 5 1/8 inches for the Lion. This meant the competing engines were long-stroke (their stroke was longer than their bore diameter) while the Lion was short-stroke (its bore diameter was longer than its stroke). As little as I understand this, this is the only design feature I can think of that would cause the detonation problems.
To fix this problem, a tank engine may have to be based on an earlier version of the Lion, as the variant that was tested was the Lion XIA, the last major variant built for commercial use (as opposed to racing use). Since the Lion was designed in 1917 before the advent of octane rating fuels, earlier versions, designed to run on low-octane WWI fuels, may adapt better to Grade III fuel as it is more similar to the fuel they were designed for. Another option would be to modify the engine to increase its stroke in the hopes that this can cure its problems with low-octane fuel. Finally, the spark plugs and carburetors should be changed to more suitable models for tank use in any case, as was done with the Nuffield Liberty OTL after it was selected.
Radials could have been run at an angle, as shown by the S-58
Or in horizontal 'Pancake' Mode
That might work, just remember that radials, being air-cooled, will require more space for airflow and cooling fans than their physical size would suggest, and that angled or horizontal layouts will require more complex transmissions to connect the engine shaft to the drive sprockets.