The part of the rudder that attaches to the boat -don't know what it's called- is corroded on one side of the pivot hole. Could this be repaired by weilding new metal and then drilling out the hole to the proper size?
That's were the rudder attaches to the aluminum correct? I'd just use a few galvanized washers over it if you don't plan on raising the rudder after every sail. You'll also want to check the pintles (were the aluminum frame actually attaches to the boat at two points). Mine were completely shot and the rudder would shack. It's an semi-easy job, just need a super long drill bit or one of those 90 degree drills.
I wouldn't use gavanized washers, they will accelerate the corrosion. Your Idea for a weld repair is good. You could try a nylon bushing.
https://www.amazon.com/Hillman-Group-58075-Flanged-Number-3/dp/B00MND8LJE