It is a fix for sure. Two things that pop up to me:
1) should have cut the mounts and plated under them instead of going in and out of them
2) Slots in the strong points of the fishplate for plug welding, that alone would almost double the strength of the repair.
The plug welds are still possible if you felt the repair is not enough. The repair as it is will likely be plenty for the rest of it's working life, but just in case you wanted to go a step further.
Find the spots with the least damage to the inner frame, hole saw just the repair plate using 3/4", 1" max size holes. Then plug weld them.
JMHO, the pics show solid welds, so you are probably good to go.