What you're going to do is burn out the windings in the motor, especially if you're putting a heavy load on it.

Rather than use 24V just get the engine wet - not soaking wet - flooded doesn't work and it should spin over. If you're using an aluminum nose cone you can get the starter spinning first then apply it to the nose. Also make sure your prop is backed up to the reverse side of the compression stroke. If your battery is any good it should turn the engine over unless you're trying to turn a large gas engine over then your best bet would be to get a 12/24V starter. Sullivan makes a good one at a reasonable price