Just welding them won't hold. I have some that were welded on one of my Doodlebug frames right under the rider for the shifter. It came out from under me and one of them bent and ripped a hole in the frame. Weld on some proper mounts so you can do folding footpegs. I'd gusset them like owend mentioned to make them stronger.
I have front suspension mounted on my CT200U frame, but it wouldn't work with the stock CT200U wheel/tire. You could run Doodlebug wheels with 145/70-6 tires. It'd be necessary to fab up a rear brake mount, but you'd also be able to run a hydraulic setup.
That's basically what I've done with mine, but I have street tires installed since this bike will rarely see dirt. The frame is the only CT200U part left on it and even that's been modified now.
Next to one of my Doodlebugs. The street tires are smaller, but not by a ton. You can also see the welded on pegs that I mentioned on the one on the left. I hate that setup and am going to have a buddy fab up a forward control style shifter setup so I can get rid of those pegs.