You can replace the ballast to the lights and rewire a bigger electronic ballast for the lights ...
Believe me Greenmagi, I went through all that thought process ... It's not worth. First, there is the heat dissipation problem, since the old fixture boxes are not meant to stand more heat (coolong fans, perhaps?), then there is hunting for the right ballast (for how much?), then where is the reflector, how is all that going to fit together, ... time, frustration ...
The setup above cost me ~$120 canadian. How much is the retrofit, ballast ...? How much time are you going to kill to make it working and look decent? How much is your hour worth?