A wartime Chieftain would need Merlin power

What engine would a Second World War version of the Chieftain use?

Probably one or two Rolls-Royce Merlins in place of the unreliable multifuel engine.

The counterfactual keeps the Chieftain hull but moves it into a period without its historical powerplant. Its unreliable multifuel engine would need replacement. The Rolls-Royce Merlin offers the strongest readily available power-to-weight option in the British wartime inventory. One engine might not be enough for such a heavy tank, so the design could require a paired installation.