1/4inch diameter of the bolt
20 is the pitch....im not sure if that 20 threads an inch...is it fine thread or coarse?
7/8 should be the length of the bolt
most hardware store carries small bolts in increments of 1/2 an inch and smaller sizes in 1/4 of an inch
you can try hawaii nuts and bolts in mapunapuna....maybe give them a call and save you the trouble of going down and finding out they dont have that bolt
as for the head ....doesnt really matter a whole too much...