If that doesn't work, use a hacksaw with a new metal blade. Take your time, it'a more about finesse than strength. You don't want to crack the toilet. Make sure your kit has new toilet bolts, or buy them separately.

While you have the toilet out, see if anything needs rebuilding. do it all in one shot. Leaky tank? You will know this if the toilet keeps running, even with new components. Or you may have rust stains on the inside of the bowl from the tank constantly leaking water.

When you're done, get an extra thick wax ring so it doesn't leak again. Worth the extra $1 or so. Depending on how long it has been leaking, you might have to shore up that floor, or repair rotted underlayment.

Hopefully, the most you will have to do is shore it up, that's the easiest way if you have access from underneath, since you said it was 1st floor. If needed, put a length of 2x4" perpendicular to the joists, screw it in to the joists, you're good to go. good luck