Your local power equipment shop should have what you need. Bring them a piece of your original cord and you want to match the diameter of the original or be slightly smaller. A larger diameter cord means less length on the spool and increases the chance of pulling the cord out to refusal. This can result in either breaking the knot at the spool or bending the recoil spring.
Your original motors may have had the original cord with stranded reinforcing wire. Modern pull cord is better for the user and the motor. It has some give to it.
Improvise-Adapt-Overcome
