I believe that engine uses a C cell, right? There should be room for a 9 volt battery.
Welcome to BPT!
My opinion on the 9volt mods from watching them on YouTube is they are too fast for the tracks and this is why I never tried one. If you are looking for additional speed with little modification I would recommend replacing the motor unit with a Tamiya Power Dash or the like. Simple and easy.
Here is a video where this fellow easily swapped out the "C" size battery in a Diesel 10 for a 9 volt without the use of solder of modifying the chassis at all. He then returned it back to its original form without any trouble. Maybe this can work for you with Diesel.
The good thing is it will just run backwards if the wires are reversed.