Ok, just thinking outside the square, in this case would it be easier to just remove the twin rails, get a 3mm plate folded up to bring it back to a single railer. Then seeing you have removed the existing rails, weld the new rail in say 5mm lower and then sit the existing motor on a piece of vibration eliminating material like this.
https://au.rs-online.com/web/p/anti-vibration-mounts/0236492? Just a thought, works well in the airconditioning industry.
Maybe even these
https://au.rs-online.com/web/p/anti-vibration-mounts/3062461
