CoreyP, I can't remember the diameter I'm sorry (about 3 cm?). There is already a metal tube and some washers in there. I took some additional metal washers for a little preload, something like 1,5 cm. You can increase the level of the oil, like that the fork will be more progressive. But not too much, as the seals would start leaking. The maximum referring the green manual 447,5 ccm, minimum 442,5. I don't know how much higher you can get without getting problems.
You may not unscrew the top end of the fork unless as long as the bike is not liftet. Even with the jacked bike the spring is under load.
When you are not sure, if the fork oil has been done during the last years, you should change the oil. Problem with the oil weight is that each manufacturer has a different idea what e.g. 10wt means... with Motul, I think 10 or better the 15 would be fine. As long as the seals are not leaking, you can keep the old ones. I endet up with the expensive ones from SKF, as I was disappointed with the All Balls seals (slip stick).
To service the fork isn't too difficult and after some years it definately makes sense. With a bit of preload and heavier oil it should be better, at least it's worth a try.
Quite interesting are these valves you can add to the old forks, available at YSS and other suppliers.