I had to do the same job on my Stealth earlier this year after my two eldest lads hit something on a distance race and lost a rudder!! Like yours, the lower gudgeon had sheared both bolts.
Started with a small cobalt drill bit on a centre-punched spot on the remains of one bolt and gradually worked up to a 3mm drill bit. I then used a screw extractor ("easy-out") on the remains. The other bolt was about 2mm proud of the surface of the transom so I decided to 'dig' around it a bit until I could get a really small pair of mole grips on it. It took a couple or three attempts ut I managed to eventually wind it out.
Repairing the gouges and scrapes was pretty straightforward and I simply bolted the new fitting on. The Stealth has a largish (5mm?) thick piece of aluminium glassed into the transom to take the rudder gudgeons.
HTH