You could remove the current "toe post" on the shifter lever and weld on an longer/wider post or an extension. It can be done with a little thought.
OR, you can get an Electronic Shifter. It attaches to the frame/motor and works the tranny lever by a push of the button (up/down). Some are expensive but the your "need" might out weight the cost. :wink:
Look at the JP Cycle site,
www.jpcycles.com for a look at one of these units. Other cataloge places have them....most are for "sport bikes", but many riders of other bikes(3-wheeler Trikes, for instance) have gone this route.
Hope this helps,
Bullzeyet