In theory it will work. The 2 main problem areas are the strength of the forks, and the steering head. When I was a kid I had the local farm mechanic build a chopper out of my Schwinn Sting Ray the same way. Changing the steering head angle, and lengthening the forks. It never broke, but I weighed about 120 pounds back then, and there was no motor on it. I would add some gussets around that steering head. It is more likely to fail than the forks. Bicycles tend to be built out of pretty thin weak tubing, and changing the rake like that puts an incredible amount of stress on it. No matter how good the welding is, the frame material itself may not be able to withstand the stress, and will eventually crack. Motorcycle choppers often used girder type forks, which were a lot stronger than straight tubes.