A helicopter will be suitable for the JFK > LGA ride. It's very close, or at least within decent distance.
However, being it leaving one airport and entering another I would allow plenty of fudge time for getting your bags, packing in and out of cars, some holding time and so on and so forth. I would really recommend you to alot a minimum of 1 hour for the pure transfer. That should keep you on the reasonably safe side.
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