You don't mean tonnes or tons you mean kilograms or kg- a scooter motor will way maybe 2kg. Just stick with that if you insist on making an axe- no extra magnets as the motors are powerful enough on their own, you only need one motor. Look in the good websites for beginners threads and there is a guide to make an axe bot- stick to that don't try and modify it!!!

If I was you I would go for a rammer or a thwackabot which has an axe which either goes over the top of the robot when you break (like saint I think?) or when to turn it goes around (like barnsleybot). Keep it simple, like Gary said you will probably over complicate your first bot and then you learn lots of things for your next bot, so don't waste lts of money and time and make sure you stick with the design you go with, even if you realise you should have made it differently half way through just stick with it so you h ave a robot, all robots could have been built better but they are bett built than when they are left in parts.