I strip them down to just the gearbox.

Carefully split open the gearbox. CAUTION: watch for projectiles due to spring compressions.

Remove wind-up spring.

Glue wind only gear in place (if present). This is in most windup toys to protect against winding in the wrong direction.

Remover any ratcheting (surplus) gears, that are not involved in the direct transmission of power. There are sometimes many of these, used to slow the spring release, and are not necessary for the transmission gearbox.

Reassemble gearbox. Usually they just snap together.
The gearbox is used in reverse, and by that I mean, the motor is attached to the original output of the gearbox, such that a step down in speed and step up in torque is achieved.

Find two springs. One small, that tightly accommodated both motor shaft and "output" shaft of gearbox. And, another larger stiffer spring to mount motor to gearbox.

Center small spring axially as best you can on motor shaft.

Place a very small drop of superglue onto shaft enclosed by spring only. I use an old wire for application. You do not wish to fill the spring with superglue. It is to be used as a transmission spring, and will allow some misalignment between motor shaft and gear shaft, but not if it is rigid. If you happen to apply too much, then take the edge of a sheet of paper and soak up the excess.

Let it dry for 24hr.

Place motor and small spring assembly onto gearbox "output" shaft. Measure larger spring and cut to size. I cut just a little over size, such that the excess can be bent back to form a flat. CAUTION: cut springs under towel or something, because "You could put your eye out"!

Once large spring is sized, attach it to the motor as well. It is not as important to be axially centered but ensuring that the motor and gearbox will be parallel when assembled.

Glue large spring to motor and let dry for 24hr.

Now for the tricky finally. I could not do this part without "Helping Hands" from Allelectronics, but where there is a will, there is a way. Line up small spring axially as best you can, at a depth of insertion onto gearbox to allow large spring to contact outer gearbox.

Then glue and let dry 24hr. Note: Excess glue on gear box can adhere exposed surface gear shafts. Do not get sloppy at this point or you'll be back to square one.

Ta da! 100 hours later, and you have saved yourself $10, but you gotta do what you gotta do. Note: Superglue works, but is brittle, so if you choose to do this, I would use the motor/gearbox in an easily accessible are, where glue can be reapplied if/when necessary, and avoid dropping them :-P