I guess you can replace the spring if you want to. I don't think that's why it isn't holding. I see no need to go heavier than stock for what you are wanting to do with it.
Kevlar, there are no fiberglass clutches. There are fiber disk, which is an organic material, not fiberglass.
Kevlar will engage smoothly. Ceramic would be a rough engagement.
I don't understand how you got oil on the plates when they were surfaced.....
