You don't say where you are, and you don't provide pictures of the knee walker you want modified. You're not making it easy for people to help you.
The folks most likely to help with a DIY project like this will be in the Alt Bike sub forum. You should also check out Adaptive Cycling (I don't browse there enough to know if they'll be helpful).
My first thought is to take one of these: http://www.kneewalkershop.com/knee-s...<br />
for all I know is what you have), remove the handlebars/stem and the wheels/steer tube/steering linkage, saw off the cross beam at the bottom of the head tube, and weld it to the head tube of an existing electric scooter. Then put the walker wheels/linkage/bars in to the scooter head tube, run the brake to the scooter's rear drum brake, mount the scooter throttle on the handlebar, and put the walker kneeling pad in place of the seat. Just make sure you picture a scooter with the right headtube size (probably standard 1 1/8 inch), a wide rear tire and good drum brake, and don't push the speed too high (maybe reduce the batteries so you can't over do the speed and you have less weight to deal with).
Last edited by NightShift; 04-24-12 at 05:09 AM.
Reason: Does the forum suddenly not like line breaks? It keeps losing my white space.