If you are planning to buy second-hand I would suggest make sure the bike is maintained properly, which most of the older karizmas are not, also make sure there are no modifications done to the bike, overall the bike had no issues and was a good bike.Using K& N filters for increased airflow and installing a free flow exhaust is the best way to get probably 10% gain on the original power rating.it very good and nice looking bike.
For further power you will have to port the engine. Which will make your bike illegal in eyes of law. Also the cost will go up and reliability of engine come down. There are very few mechanics who can port perfectly. So not a very good proposition.
Better to purchase another powerful bike rather than porting engine.