I had the same problem with mine when I bought the boat. Someone had actually epoxied over top of them. They are aluminum plugs and very soft. I found a 8pt. socket I think to fit the plug and put an extention on it after heating up. I moved it some back and forth working it out eventually. The threads were still ok.
After what seemed like 100 gallons of water pouring out for a few minutes, I was able to replace them with a temporary PVC plug which was 1/2" pipe thread size. I eventually found some aluminum replacements from Ebay and have used some grease on the threads to prevent leaks and corrosion.
I have been removing them after every outing for the first year and continued to get water draining out until I finally got it all sealed up. Now, I remove every few trips and only a few drips come out, likely from condensation.
It is possible to drill it out, and thread it. Would start with maybe a 1/4-20 thread and SS bolt so that if you make a mistake, you can go to next size up. But, hopefully, you can get it loose.
Another thought is to drill the 1/4-20 hole, squirt some penetrating spray in there and see if it can soak for a while and help remove the plug.
Good luck!