If this is a question about possible impacts on heat treatment by welding, it would be good to know, what the factory procedure is. Do they perform a heat treatment after the engine mount is built, or are they done? Vans could probably answer that. Knowing that it worked for someone else is nice, but better still to know from the folks that designed the engine mount that mods don't interfere with strength.
I've been looking into a similar question for Cessna spring landing gear. There are instructions to avoid removing the surface shot peen, warnings about paint removers- hydrogen embrittlement, and finally warnings about welding (definitely not). I came across pictures of a step that had been welded on instead of epoxied. Gear broke at weld. The high heat of welding created a very weak area.