Nice SSSCAT-II. The original tires spec'd in the manual were 4.00 x 6.00 (4" wide, 6" rim). Funny how many ways there are to call out a tire size. 14x6, 530/4.50 and 530x450x6 are basically the same thing. 14x6 would be closer to original but I typically use the 530x450 Chen Shin Trials. They are darn close and only $20 vs. $55 for the 14x6 reproductions. Just don't use the 410x350's, as they are too small.
Regarding the front forks, yes, it's a split nylon plastic insert. You'll need to remove it to clean them thoroughly. The bottom end sits in a shallow groove. Use a small flat screwdriver and a pair of needle nose pliers. look in the end and locate the split. Pry up carefully just to the right or left of the split. Get a hold of the corner with your pliers. Twist the corner in on itself, like you were tightening a roll of wrapping paper. As you do that, start pulling the insert out of the tube. You might need to use a couple of small screwdrivers as wedges to keep the plastic from popping back into the groove.
I've got a few parts that will help you out. I've got pegs, shocks, headstock bearings, brakes, levers, probably a set of jack shaft bearing clamps, reflectors, and tail light. Most is NOS. You can probably repair the bent shocks if the chrome is good. I do it all the time. I have the correct headlight for this bike too, which was an odd mount on this model and very tough to find, but it needs re-chrome and a new bulb. Let me know what you need once you get your list together.