Before you try the more extreme and/or expensive solutions, try cleaning out the area between the fork and the fork seal. Sometimes grit can get in there and cause leaks, but if you get the grit out, the leak will stop. You can make a seal cleaner out of a piece of plastic milk carton. There are videos on youtube on how to make a cleaning tool, and how to use it.
I agree with what RCinNC wrote here.
When I started riding my new to me 2013 bike last year it had seen little action over the latest years and had just short of 30000 km on her and as a result of that both forklegs had developed a severe leak that made me look into my options....
What I ended up doing was following this guys instructions and my fork went dry in a matter of minutes.
I had always heard of the trick with feeler gauges but never had to try it and although I believe it to work well the curved shape of a seal mate original or the copy made from a PET-bottle I think makes it really easy to be kind to the seals.
I also believe that the hooked tip of the "sealmate" helps to pull out whatever needs to be pulled out instead of pushing it into the forkleg.
I really think you should try this if your fork feels otherwise healthy.
