There is a company called "Pyrimid" who makes the single spring (girder) front fork
in 26" as a bolt on. It's a straight replacemen and they supply them in chrome or paintable
black if you wish to paint them to match your bike. (those forks which use multiple springs are called "springers" from what I've leaned here)
I've seen them on fleabay for $40+ up to $70 so ya gotta shop around. (pay attention to
bottom line cost there.....item + shipping = total, as some offer free shipping with "buy it now"
which may be a competitive price.)
And of course, always keep moving parts lubed properly, and inspect for signs of stress cracks
or unusual wear. Riding a bike with a motor for longer durations may stress it more than what the
manufacturer anticipated when they chose the materials to build it with. In Ohio where I live the
speed limit on motorized bikes is 20 mph.