well if the bolt sheared off you are going to have to get the rest of the bolt out before it can be replaced. so I am sure there is enough bolt left to measure it to find out with it is
I am more then certain it is meteric, to measure take the diameter and that is the first number and then measure the distance between ten threads, then move the decimal left one spot, that is the second number the pitch
so as an example M10x1.5 is a 10mm diameter bolt with a 1.5mm pitch and then you just pick the length that you need