I agree with Paul, it will be much simpler to replace those gear changers than try and modify them.
I prefer the more natural action and cleaner look of the twist throttle control:
I just wish the grips were shorter, or at least went completely over the handlebar instead of sticking out an extra 25-30mm on each end.
But, if I was planning on using a thumb throttle, I would fit of a pair of these:
And a 11-30T 7 speed thread-on freewheel:
The 30 tooth 1st gear sprocket is not likely to be used much
(if at all) but the 11 tooth 7th gear sprocket will be a necessity if you want your legs to be able to keep up with the pedals while the motor is running at full speed.
Here's a handy little tip for anyone using the thumb throttle:
Find a couple of coins or washers which are the same diameter as the handlebars
(~22mm) and pop one inside each of the rubber grips before pushing the grips onto the handlebars. If your bike falls over, they should prevent the handlebars from punching a hole right through the end of the rubber grips as they hit the ground.
Alan