I have converted several high gain band specific mobile antennas for base station use by fabricating mounting plates with attached radials but a mag mount? Uh uh, for more reasons than I care to list as they say in your neck of the woods, ***etaboudit. All things considered the best single antenna I believe would suit your purpose is a discone but beware of flimsy construction outdoors. I'm not a big fan of Rat Shack for that reason, Diamond is a bit more expensive but well worth it staying together.
Tooterville SI is a great location since you have a great line of sight over water to LI and NJ, just be aware of the fact that high elevations cast radio shadows. You'll have problems with Todt Hill, Mount Mitchell, Telegraph Hill and the Hudson Palisades so don't complain, like me you're at sea level and I have similar blockages up north.