Do they need to be beefier? The box section is about 2mm thick and the plate is 5mm.

Cheers.
Steve.
Rescued attachment DSC02407.JPG

I would say that the box section looks a bit weedy imo, especially as the mount is quite long.

2" square box maybe? 5mm plate seems alright though

with the length and angle i'd beef it up a bit

I used 40mm square box with a 3mm wall to make my engine mounts

bottom one needs a curved fillet to the top of the bolt tube. Top needs a bit more thought IMHO, nice cutting at least! Couple of fillets to spread the load should sort it though.

I would try to add a brace going back to the rear of the engine making a V shape if there is anywhere to fix it to. If not then the box section needs to be bigger IMHO. I think that when the engine rocks it will eventually break the mount.

The mount is flexing as it is,
it would be better straight down.
Reposition the lower mounting plate that way you shorten the mount and make it work in compression