Are you absolutely certain it has no turbocharger?
If so then there really is very little you can do apart from the obvious ie. clean air filter, clean fuel filter. Fill it with decent fuel and run a charge of Forte Diesel Treatment through it. Clearly these steps will help with any engine.
All this will do is get it as close to standard as possible.
The sad fact is that naturally aspirated diesels are virtually impossible to tune. Increasing the fuel delivery won't help as there will not be enough air to burn it completely. That's where a turbocharger comes in handy. You can make a 2 litre engine breathe like a four litre one.
You might just be able to retrofit a turbo but the cost will be horrific as you'll need different manifolds and exhuast, different injectors. probably even different head and pistons to lower the compression ratio.
And if you did modify it to this degree then insurance could be a challenge.
Im afraid that naturally aspirated diesels were never really conceived for their performance capabilities. What they are very good at indeed is turning in exceptional fuel economy figures.
So you could look at this as a means to save up quickly and replace the car with something livlier altogether (which is what I'd do), making sure that you plug the fuel economy of your Di in the advert and get a really good price for it.