Id have a piston for it. you can use any 74 yam piston yz/mx/dt even, be it curved window lower or the 2 window model, only check you have the right rod in it otherwise it will sit 4-6 mm up when using the later pistons.
i would also have rings for them if he is just doing a freshen up. but you would need to say if your using a 2 ring or single ring piston.
These bikes are the easiest to keep the motor alive.