Is the excess of current the only problem in a motor overvoltage?
In addition to the obvious mechanical problems with bearings, if it's a brushed motor you will quickly wear out the brushes. If the brushes spark, you can mitigate this by rotating the brush assembly (on a motor that allows this, most small motors don't) to minimise sparking at a specific speed - provided you don't need to run the motor in both directions.

However, such sparking can quickly erode brushes and commutators at which point the motor runs inefficiently and accelerates its failure. On big motors, commutators can be machined to restore smooth operation (until they are completely worn away!) Carbon brushes will take some abuse and are usually replaceable, metal brushes may simply melt in the arc...

If you're OK with shortening life to a few hours, a moderate overvoltage - depending on the motor, maybe up to twice its rating - is survivable. Perhaps acceptable for a toy, definitely not for a vehicle.

This is the main reason why high performance applications have moved to brushless motors - if the drive electronics permit, the bearings become the main limiting factor in speed.