Asus ROG Strix FX753VD motherboard heatsink screw and nut sizes

Hi, I've bought a faulty laptop that was said to be overheating. the fan outlet was blocked with dust so cleared it out which improved things but it still overheats 30 seconds after starting a game.

After removing heatsink found a screw under the heatsink on one of the gpu memory chips which wouldn't allow the copper to touch the gpu fully. So when to refit and realised the person had tried to screw the the sink down that hard it broke 2 of the standoffs from the motherboard so i can't screw the heatsink back down.

Only hope is getting a long screw throught the motherboard and a nut on the end to hold the heatsink to the board if that makes sense. Picture 1 is the screw sitting next to the heat pad, picture 2 is the broken standoff, picture 3 is the hole in the motherboard i'll have to try to secure the heatsink down to.

So any idea where i can get a long screw and a nut to go on the end to hold the heatsink down?