I don't know what diesels are available in Australia. Over here we only can have either 2,4 liter free aspirated, or with a turbo. That's it.
As much I generally respect Toyota engineering, I have to lift my hat to German diesel engineering this time.
Especially VW group has pretty phenomenal diesel technology, low consumption with high power output.
Back to your friend's HiAce.
I suggest replacing the 2,4 liter Toyota unit with a larger displacement direct injection unit.
There is plenty of diesel tuning information available all over the internet. Especially on american web sites.
Info from such sites is directly applicable to any diesel - in principle. They may achieve modifying certain parameters with certain parts, whereas Mitsubishi/Toyota diesel enthusiast must use his expertise to source equivalent component to his engine.
Around 160hp power at wheels per 2 liters of displacement is very well achieveable figure with a turbocharged direct injection diesel engines. More if tuning gets serious.