I would recommend buying a new stopper grommet. The old one may not be usable depending on how old your current tank and stopper is. The old one can be pryed out if done carefully. First pull out the stopper and clunk line. Then pry out the stopper grommet.
Depending on what clunk you use you may need to put your fuel line through the grommet before reinstalling it. You should be able to insert the grommet into the new tank opening if it’s still in decent condition.
Cheers
Alan