First, you should have someone who is estute to structural engineering take a look at this. Why, you ask? Because if enough of the wood is rotted and infected to the point that even if you were to 'route-out' the bad wood, is there enough left, basicallyto support the house!!!? I don't know if epoxy is sound enough to support the weight that the 'routed-out' wood use to support. You see, as part of the clean-up, you would possibly need to support the floor so that if there is too much rot, the floor doesn't collapse/sag (where you'll end up having to jack up the floor to replace the sill anyway if it does collapse a bit.)
So, I suggest you have a professional look at it even if the damage is not that extensive to the inexperienced eye. If you don't know where to get someone, I suggest you call a Home Inspector company that's, at least, ASHI approved for recommendations of folks that are engineers or structural experts who are experienced with your type of problem. Tell them you will want info to see if you can do the repair work yourself. Personally, since I'd like to keep my 130 year old house in 1 piece, I'd pay the $$$ and have it first professionally inspected, then, talk about repair or replacement.
Before I forget, I remember seeing this wood epoxy used on window frames and such where there was either little termite damage or water damage. The damage was 'routed-out' then treated, then epoxyed. I couldn't tell you if epoxying a sill is a good idea, one way or the other because of the structural-support issue. My angle is from a safety angle and a do-the-job-right angle (when it comes to a 130 year old house and its sill.) Good luck. Please let us know what happens so we can all learn.