Check under that recover on the seat, you might be surprised to find what was on it before. One of mine had something similar done and the original seat was still underneath it. Although not in great shape, I was able to pull a pattern from it and build a new one.
As for the wheels, It could have had 4" or 6". Sometimes it was dictated by where the rear fender strap was located I believe. At least on a couple of mine you can't put 6" wheels and fenders because the tire rides to close to bracket. If the motor plate is original, it appears that it would have been a Briggs bike. There would be a small round hole towards the center if it was a Tecumseh bike. Anyone with a LiL Indian can give you the dimensions and location of the bracket. I can get it for you if nobody has it handy.
As for the forks, it looks like they've been racked slightly. It's probably putting a bind on the bolt itself. The lower triple tree looks like it's out of whack compared to the top one in the picture. A piece of brass and a hammer might help you get it out from the bottom. That way you don't mess up the bolt if it's still good.
Hope that helps.