First off, clean your intercooler out. You can find plenty of threads about doing this, and it does make a big difference.
There are a few things that can be added to a Disco 1 for more go:
Sports exhaust - if you want to fit a side exit one then you can delete at least the back box. I have heard a 300TDI with no silencers at all and it didn't seem obnoxious to me, but MOT testers may disagree. You can also get rid of the cat, but I'm not sure if this is legal to do on post-94 vehicles?
Blank the EGR valve off - doesn't give much (if any) extra go, but it will help your engine breathe more easily
Tweak the fuelling - do not fiddle with this unless you've read the various guides out there and are confident that you know what you're doing. That said, as yours is a 1997 model I suspect it may have the electronic "fly by wire" throttle, which this doesn't work on. I'm not sure if you can buy a chip for these?
Fit a bigger intercooler - the full width front mount ones will give you a lot more power
It is possible to talk a 300TDI into producing around 150bhp without too much hassle, and without much risk of melting anything. Don't fiddle with the turbo boost pressure on a road (as in non-competition) car as that is a very quick way to wreck the engine!
Edit:
Before doing ANY of these, you should check that your suspension, brakes and steering are up to it. The reason for a lot of older Discos handling like jellies is that people don't bother replacing springs, shocks, bushes, etc. A good set of polybushes and gas shocks will do wonders for the handling, as will new springs.