Replace a piston or the entire motor?

I know someone that has a 92 Jeep Cherokee with a 4.0 for sale. One of the pistons is cracked. Would it be cheaper to replace that piston or to replace the entire motor? I know that the best choice would be to replace the motor. If I replaced the piston, I would replace the motor later on. The only reason why I'm looking for the cheapest because my parents will be buying it and I'll be paying them back. That and because I'm doing fill in work and don't really know when I will work. I'd replace the motor after I got a normal work schedule.

I've been in that position before; I would do the bare minimum and drive it gently until finances permit something better. BTDT too many times. Just remember that if you beat on it you may well end up walking
Joe