Modifying or attempting weld and machine a small camshaft such as those used in model engines would be difficult.. Many reasons for this assertion. One could offset grind the camshaft [so to speak] from its intended base circle by turning down the outside diameter of the pins or journals located at either end of the camshaft typically supported by bearings. Then simply replace the bearings with with new units sized to the new journal size. This would effectively provide more lift to the valve train. At a minimum of effort without compromising the camshafts metallurgy and or messing with the profile.

OR:
Changing the engines rocker arm stand-off installed height would provide similar results.

OR:
Modifying or fabricating a new rocker arm would provide similar results.

OR:
Shaving or milling the head and or cylinder Assembly would provide similar results.

OR:
Longer or taller push-rods and or valve lifters would provide similar results.

OR:
Or longer or taller valve stems or possibly lowering the valve into the head slightly would provide similar results.

BUT:
With all these considerations one must consider and observe weather or not valve spring coil bind is or maybe present. And lets not forget about piston to valve clearance...

And after all these secondary methods, will increased lift without thought for duration effectively change the engines performance? Cam profiles have several specifications, lift is only one of them. Duration, ramping and timing are keys to increased performance.

Larger valves maybe of some benefit without all the other headaches...

Or how about simply using higher nitro content or purchasing a YS Engine if you really feel the need for performance with a minimum of effort.