depending on the condition of the hoses, you could try to pinch a section of hose on either side of the motor with forcep-like pliers to stop the flow of coolant when you remove the motor. you will still introduce a small amount of air in the line when you do the swap, but the system will be able to purge that out by itself automatically after a little use. considering the age of the hoses, you'll have to be careful not to damage them when you 'pinch' them. In fact, what you might even consider is getting a helper to pinch one side by squeezing the line hard with just his/her fingers while you disconnect the fitting on the motor assembly. When the hose is off (their still pinching it somewhere a few inches away), you could quickly plug the open end of the line with some kind of stopper, like a cork. Repeat process on other side, take out old motor, get new motor, reverse process. You'll lose a little coolant, but if you're fast enough, the amount of air you'll introduce to the system will again be minimal enough that it will purge itself after restart.