Ok, it is possible to get more umph, but it involves some spannering on the plain 1.9d and is limited.
1. drill out all the little blanks in the grille - in front of where the air filter feed is. Cone filters are sh*t so just fit a k&n panel filter and make sure the airbox is tightly clipped & sealed afterwards - makes a horrible drone otherwise.
2. Good exhaust condition is necessary, not a sports one but a tightly sealed oem one with no leaks (you lose bottom end torque with a leaky exhaust)
3. Find out what injector cc is currently fitted and fit larger injectors (next size up) - the only caveat on this mod, which will produce a big power gain - is you can't increase the airflow much as its NA (no turbo) so if you get excessive smoke then you would have to go back to the oem ones. If not, then hey presto, more grunt. The standard injectors don't make smoke so all the available fuel is getting burned....if the next size up makes little or no smoke then happy days, you will get more power without failing an MOT smoke test - but check under load and at full revs just to be sure (when engine is fully warmed up) - breakers yard is a good source of injectors or ebay.
Alternatively find the max-fuel screw on the injection pump and tweak it for max power output with acceptable smoke output i.e. when it smokes, back it off slightly.
More fuel = more grunt on any diesel, its just a question of finding the sweet spot when almost all the fuel is burned with a teeny bit of smoke left over.
Hope this helps, just my tuppence worth. I'm more used to tuning vw/audi tdi's cos thats easy