starters not engaging

Gcranor: I'll just jump in here and assume you have the 6 volt version of the starter, Delco number 1107041. If the Bendix drive spring broke the drive assembly number is 1873789. This is available from Rock Auto or Ebay. Confirm pinion tooth count before ordering. From the Delco catalog and pictures it appears on this drive the whole assembly is replaced, you can't replace just the spring.

Caution note of running 6 volt starters on 12 volts: The internet is full of good and bad information and sometimes we have to pick a side. It seems the majority of folks who offer advice on converting their rigs to 12 volts are satisfied with using their 6 volt starter as is. Some have commented that they have run that way for years with no problems. However there is another side to this story and if you're interested this YouTube video demonstrates the downsides: 6V and 12V starters https://youtu.be/jn7d-KZsIxs
It also shows a 1950's era Delco service bulletin warning of mechanical failures from the voltage increase.

The 50's Delco catalog I have also shows that two different 12 volt starters were offered, probably aftermarket, for the lightweight K and KB trucks with GRD engines.

Tom

I would disagree with some of that video, series would motors will not turn twice as fast on double the voltage, but the torque will increase considerably. Many starters use the same armature for more than one voltage. The (series would) field coil limits the current (and therefor heat) in the motor. The smaller Delco's may use the same armature for 6 and 12 volt, I know the larger ones use the same for 12 and 24 volts.
Those with the mechanical starter button (linkage) have the pinion in the ring gear before the motor is powered, so it will not bust the gear off from spinning, but the torque of the motor will be almost double from stand still and may be too much for the drive.
The larger K's use an inertia type engagement drive and would be like Ford tractor starter.