Years back I used to occasionally stamp serial numbers and other information as part of my job in aerospace manufacturing. The stamping didn't have to be perfect, but it did need to be reasonably neat. We used to use Harbor Freight stamps to do this. They're not the top of the line, but they work well.
If you want to keep the lettering lined up, create an edge with masking tape and carefully move the edge of the letter to the edge of the masking tape. Doubling the tape up may help. Practice on a piece of scrap metal first.
It may be more authentic to eyeball it, however, so if you want authenticity, you may want to look at some others to see how well they line up.