Is there a way to debuild the jar file and then rebuild it using the newest java version?That's not necessary. What is necessary is to find out why it doesn't execute. Most probably you don't have Java installed at all, or correctly.
I also tried to ask help from the creator but they are cluless.Surprising. That being so you should evaluate whether this thing still has any value for you, and if so hire someone who knows what they're talking about.