Most reefer motors are designed for intermittent use rather than continuous use. A good reefer will only run for a few minutes per day. The motor needs to do some heavy work for those few minutes (run a compressor).
That being said, an old Kelvinator probably has a much more durable motor than a newer one would.
(Corporate logic says "If 99% of our motors outlast the useful life of the appliance, we could get away with a cheaper motor.")
Since the unit is now out of service, I would say to go ahead and use the motor. If it burns out, what have you lost?
If the reefer is old enough (no plastic inside), they make wonderful smokers.
Or, just save the shelves for herb drying, fire pit grills, etc.
I recommend looking into smog pumps from automobiles, in particular the one from 1988-1997 Ford trucks and vans with the 7.5L engine is a big unit with it's own electric motor.. I've heard of them being used to aerate ponds