Firstly, I don't think your plant can possibly be m. gigantia with those dimensions. Gigantia will eventually make a plant much more wider that your 2.5 inches, growing to 6 inches or more in breadth. At the same time not increasing it's height by very much. Also, the flowers will be green. If you were to post a picture here we might be able to give you a more accurate name. I think KW has a magnificent plant of m. gigantia in his collection, so he might be able to put a photo up for you to see.
Secondly, KW's suggestion is spot on. If your plant was of a type that might offset in time, which clearly yours hasn't after 20yrs, a hanging basket would be another option.
Look forward to hearing from you.