Nuffield/Leyland diesel tractor four cylinder might be a valid option if wanting to keep it kind of original or at least as an example of what might have been. The 3.8 as found in Leyland 384 or the 3.4 as found in Leyland 344. Not sure of Nuffield models but fairly sure most parts were interchangeable as those first Leylands were pretty much a Nuffield with different bodywork.