First choice: Bigger engine. Don't need no steenkin' dead weight.
Distant second choice(s): Bigger radio battery, as far forward as possible, along with anything else that ca nbe moved. Spinner weights are good for maximizing moment, but only for a couple of ounces.
AMA Safety Code sez no metal props, so when you've run out of options,
Last choice: A chunk of lead from the plumbing supply, or mix up some BB's and epoxy and pour it into a plastic cup, modeling clay mold, etc.
Abel