The cool thing about tweaking a smaller bike is every little bit you change can be felt in a big way. Take the snorkel out of a ninja 250 and stick a few washers under the needles and INSTANTLY the bike feels more powerful. Get a lighter exhaust system and the weight savings are reflected in the way the bike accelerates. Tuning smaller bikes is fun in its own way.
Having said that, I think you may be hard-pressed to find decent parts for your bike since it was never officially sold in Canada or the US. You can import stuff from Japan or Australia but your looking at ALOT of $$$$. Unless your a collector or just HAVE to have a 250cc in-line four, get something with more parts readily available or get a bigger bike.