@Nancy Hart maybe there is a dealer or mechanic in your areas that can answer your questions. You can get those plugs. I replaced one like you have, but I could only get it with a square to, not an Allen/hex top, but it worked just fine if the size and the thread is the same.
Postscript Cold and rainy today. Got the top plug out with a 7/8" socket. Not deep enough, but made it work, and found one cheap on the way home, at a little farm store in the country. The oil inside is only about an inch low and looks clean (straw dipstick). Feeling better about this mower now.
Glad to hear it, @Nancy Hart ! You are contemplating mowing, and I am worried I am going to have to break out the snowshoes as the snow is getting deeper and deeper.