As I look at the pictures, there are some it seems where the mount has the studs removed. The picture where I see the stud in the picture, it seems to be bent to go around the bike frame tube.
If this is what I think I saw, then grinding or what ever to make the mount to be the larger radius to fit the frame tubes may not be all that is required. Having the studs being bent weakens them and also puts pressure on the bike frame tubes in a way that was not intended and also may weaken the tubes.
If there is enough room in the frame to make a kind of adapter bracket, that goes between what exists already on the engine mounts, and then exactly fits the frame that could be an idea. Only that even if that could be done it would be ingenious to be done without special tools. But not that it cannot be done.
For example I have a motor bike (not really bike any more as no pedals), where I had mini bike foot pegs. Then I wanted more grip using fold back motorcycle dirt bike foot pegs. To get the mounting at the correct angle I welded some metal and drilled holes. Bolted the adapter in place and did not have to resort to welding the newer pegs on permanently
I looked at the two screws that holds the case, but also definitely the place that the studs comes out at 90 degree angle to it. If those two screws have enough strength to be used in some way to add and adapter here is an idea. What about using two longer screws to fasten and adapter that could have the studs mounted a bit further separated. This way they clear the frame tube without touching and bending that may be an idea?