That's an oldie.
To begin:
Look at the overall mechanical condition, determine what needs maintenance and repairs
Anchor the machine to the floor, level and square.
Take an inventory of the existing axis motors and drives.
Look at the existing controls and see what can be reused.
Remove all of the gas torch hardware
Buy a Hypertherm Powermax 105 or equivalent
Buy a new control system, many vendors are available, from really cheap to crazy expensive.
Looks like you have a project there. Pretty easy to get it going if it's mechanically OK.