You can buy camshafts with welded and ground lobes to increase lift and duration.
I would not reccomend it if you do not know what you are doing. They use proper camshaft grinders with known and tested profiles.
Also it may cause piston to valve issues, and actually hurt performance.
One thing that is possible is grinding down the base circle to increase lift. This does not modify the lobe and you are less likely to hurt performance.
What I have heard:
Increase lift= more torque
Increase duration= more RPM.
However, there are lines you do not want to cross.
Another option depending on what motor is to buy ratio rockers, which increase the lift at the valve. For clone motors they start at $20 for a pair.