Sierra cast alu engine mounts

[Talonmotorsport]

I personally would have just ditched the cast mounts and fabricated new ones from 30x30x3mm or 40x40x3mm box section and plate, but then thats my answer to every thing if it doesn't fit don't spend money on it just make one.
The only problem I can see with your cast part is that where you now have a 90' machined cut at the bottom this is where it will most likely break in the corner. I would have removed most of the material with the end mill then done the last section with a bull nose cutter to give a 5-10mm rad so there would be less of a stress point.
All you can do is run it and see what happens you may get away with it for awhile or may never break only time will tell.