The big thing is the two holes that water moves through. The large one is suction the smaller one is backpressure. The gasket would have to be able to cover both holes individually without any leakage between the two. The holes in the casing adaptor can be filed with a rat tail file to make the bolts fit. I have done it many times with Sta Rite and Goulds adaptors. They are similar but not identical. Welding won't work. Making an adaptor I suppose could work if your good at fabrication.
Do not try to remove the casing adaptor unless you are extremely adventurous. Remember there is a pipe inside of a pipe. Called concentric piping. The inside one is probably Poly pipe. Getting it off isn't too bad, getting it back on so it can function again is another thing.
Take a look at the two motors. F&W used a C-Flange motor. The only other motor that 90% of pump companies used was the square flange. So if your two motors are either C-flange or Square Flange, you can just change the motors. That is if the horse power going on the old pump is the same or higher horsepower. They all turn 3450 RPM's.