Fixing A Springer Front End

[Various]

Bike gurus:

I bought a tricycle with a springer front end cheap on Craigslist, and in trying to tune it up, the main springer bolt (what I would think of as the kingpin on a skateboard - the bolt going through the spring to the collar) sheared. Assuming I can get the remaining rusted bit out of the collar, can I replace the bolt with something from the hardware store? I've heard rumor of the bolts being some proprietary size that's impossible to find except as expensive bike gear. Any idea of what the bolt measurements should be?

If there are many types/versions of springer fronts, I can upload pictures of the unit.