IF you can dry out the rotted part, use epoxy (I use WEST) thinned 1:1 with acetone. This will permeate rotted wood quite deep and set up a sort of an epoxy matrix. Then use epoxy with sawdust and/or microbaloons to match the surface finish.
This will only work if you can thoroughly dry out the rotted section. I use it for boats, and bring them inside with a heat lamp on the affected area for at least a week.