You could get some cheapo poly body mounts and replace them and see if that helps. Are you sure they're put together correctly? Should be a bushing between motor mount and frame mount, bushing below frame mount, big washer below this, metal sleeve through both bushings, bolt tightened to the sleeve. Works fine for body mounts, don't see why it shouldn't work here, and you wouldn't have to cut it all off and plate your frame and make new ones