I had a stuck spark plug the other week, ended up putting the impact wrench on it and it literally only went "ugga dugga" once and it let go without damage
The other thing I would say, try new lines and primer, see if it will get running with the current stuck plug, get it red hot and try pulling the plug then
Funnily enough I was given a tm251sst yesterday - was used once in 2005 and put away - like new. New lines and off it went, great little trimmer