personally i would replace it. the whole swingarm. they are readily available on ebay from 30-100$ i dont really think itd be a very hard job. im not sure, but id bet any kind of possible repair like welding in the hole would cost as much. but im not sure id want my life riding on a swingarm with a "repaired" hole in it.
im not expert but its an obvious weak point in a piece of kit thats been engineered to be as light as possible yet strong enough to do the job.
you might be surprised how much flex goes on under hard riding condition on a undamaged swingarm. if it were to buckle, itd be likely the worst possible time.
like i said drop 50$ on a good used one on ebay from a reputable seller with a decent feedback score and the option to return. then make a weekend out of taking the old one off n replacing it.
itll require a new masterlink for the chain and the tooling to install it. or just a new chain if your close. ml are like 15$
i think there is a tool required to remove the pivot bolt, same tool used to "hold" the front axle if im not mistaken. also a 10$ item online.
then just read up on the service manual on the removal and install. service manual download can easily be found online.
i suppose if it were a small hole, like no biger than 3/8" id consider having it filled by a professional welder, but if its any bigger, and i assume it is, id replace the whole thing.
the risk far outweighs the cost of 100$ + a few hours of your time to fix it. personally i cant ride the way i want to if im not 100% sure and confident my machine is in top shape. something i wont compromise on.