Stripped Thread - size anyone?

[Eightballz]

hey there,

i'm in the process of rebuilding the 455 of my 1970 toro.

last weekend i tried to mount the rear oil deflector but looks like i stripped the thread on one of the nuts (see pic)

so...can anybody help when it comes to the size of the thread?

want to order a new nut and maybe a thread cutter..don't know if the thread on the stud is stripped too.

or...how important is that 3rd nut? i mean, that nut would'nt be there if its got no use.