I don't recon that there is a diesel ready module in matlab simulink, I think better yet you should proceed with objective oriented/goal model, what is the target of your model, are you going to use diesel hybrid and you need transient response or are you just model the dynamic or static response of the system,

However, there is nothing easier than modeling a diesel generator in general, it is just a matter of determining the time constants and comparing them with real cases from actual generator using parameter estimation tools, here is a sample in the photo and there are too many paper of open access available on google scholar for your subject , I have uploaded those photos from the following paper:

https://pdfs.semanticscholar.org/9830/a4964270f774bbf78d6dee698d8fd9b4fc16.pdf