I take it the bolt threads are intact and not stripped, measure the bolt with a vernier caliper across the threads you then have your bolt size then measure the thread pitch on the bolt with a thread pitch gauge, then with these two measurements you can determine what size helicoil kit you need it will come with drill bit, thread tap, the right thread pitch helicoil for your bolt and the helicoil insert tool
As TRO says, you should be able to measure from the bolt still as the damaged female thread was probably aluminium. Metric threads measured at the bolt will be slightly under their nominal diameter. Yours will probably be an m8x1.25mm and the bolt will measure just under 8mm across the threads. If it is all trashed and you are not sure then please post a picture and I will measure the undamaged one in my garage and link to the correct helicoil kit you need to buy.