If I don't have the keys, I just drill the lock and then put in a keyless lock later. Just center the drill bit right on the hole, and drill straight in. You only need to drill out the mech, not the entire lock, so the bit you use only needs to be a little bit wider than the keyhold entry itself. Center it on the lock, drill straight in until you hit the back of the lock, and pull the drill out. Stick a flathead screwdriver into the hole and twist. It should turn for you. If not, try stepping up to a little bit bigger bit.
Drilling a lock shouldn't take but a couple of minutes to do.