If you or your son has any mechanical skills tools (and time) rebuild is the best / cheapest option. The 350 IMO is one of the rock solid motors.
Your looking at $200-$400 on rebuild kit and probably $400-$800+ (depends on why cylinder #6 is low) in machine work.
crate motors are around 1800-3000 depends on what you want.
Or go to a junk yard and get one!!!