I don’t think strength would be much of an issue since these brackets don’t really get the same abuse as a kickstart lever would. As long as they support standing weight you should be fine. A decent-sized washed on the backside to displace the stress would be helpful.
As for drilling the holes, buy a reamer. You can get them in exact decimal sizes. When using it to size a hole, never reverse the bit to extract it; always leave it running in the cut direction and retract the bit. Otherwise you will dull it immediately. A small chamfering bit front and rear to center the reamer will make a very professional result.