I just saw this video by White One Racing, a couple of Italian guys who do some interesting stuff.
This is a subject that has popped up from time to time - namely the fin design of most Piaggio and third party heads actively restricts flow to the outer fins. This is partially due to the design of the cylinder cowl. Air flows into the channel from the flywheel and arrives at the head, where it "sees" maybe 5 or 6 fins. The air flows over these and out.
The side (shorter) fins are more or less excluded from this process, and I'm guessing only really cool by transferring heat to the centre fins.
I like the idea but wondered if it might be easier to alter the cylinder cowl instead of the head itself - he does mention in the video that you shouldn't cut too much or risk making the cooling worse through lack of area.
I haven't had any luck with plastic welding, but maybe all it needs is heating and shaping to make the channels rather than adding any material.