some other posts have suggested stuff (including the above stuff,) like BBs glued in place, fishing weights, car tire weights, I've even used pennies. I didn't have access to GRJ's bullets! Hot melt and/or double sided foam tape also works.
I'd agree with moving the main board to the tender if you could.
Lastly, I have cut, shaped and moved around the original weights to gain room.
It's nice to hear that there's an O scale 3 rail loco out there without tires on it! Hey, I never knew.