I've retested the script, and I've had no problem with respawning choppers.
The thing is, if there's only one type of chopper in the air pool of a given side, there can be no more than one chopper per group. To remove this feature, which I did for land vehicles but not for air ones, remove the following lines from the script under #SpawnAir :
_OKS_count_vehic_temp=0
{if ((vehicle _x != _x) and (_x==driver vehicle _x)) then {_OKS_count_vehic_temp=_OKS_count_vehic_temp+1}} forEach units _grp
? _OKS_count_vehic_temp >= (count (OKS_global select _j+12)) : exit