Good find on the stamp guy. I've seen Lionel stamp kits being offered on ebay, but I've never fiddled with one myself.
I suspect the quality of the finished lettering is due not only to the quaility of the stamp, but the skill of the person using it ... steadiness of hand, exact right amount of paint to ink stamp with, etc. I'd bet this route would offer great results in talented hands, and iffy-crummy results in unskilled hands. An "art" to it, so to speak.
Along that thinking, I admitted to myself that I had no such skill. So when it came time to reletter (or renumber, really) my Lionel 221 loco, I opted to use Woodland Scenics dry-transfer decals. Easy to apply, very crisp edges, and absolutely no clear-film margins. I'm quite happy with the result ... perhaps the only criticism is that the numbers might look "too perfect" as compared to Lionel's true original-production stamped numbers. Crisp? Absolutely. Authentic? Nope.
TJ's 2-cents ...