Before you re-rivet the tiller arms, put some epoxy in the socket to stop it coming loose again
What he said ^^^
I've had them riveted on top and bottom, side and top, bottom and side, wrapped it in plastic, made spacers, wedged crap in it...they all eventually came loose but thickened epoxy and two bottom rivets served me well for multiple years with no problem.
The epoxy solution would make repair a little more difficult - but epoxy eases with heat and all you would need to do would be to cook it for a little while to get the epoxy to let go of the aluminum arm.