Hi Boo,
I have a Eureka (like Andrew I didn't make it).
Mine looks to be the same as the one published in Ivan Law's "Gears and Gear Cutting" but was made for 19mm (or maybe 3/4") bore cutters and I modified it to take 16mm. I don't think you need to scale everything.
I'm not keen on machining gauge plate - it's certainly a lot more difficult than mild steel. There are only a few bits listed that require hardening and the book suggests case hardened mild steel. Hardening and tempering gauge plate is (to me anyway) easier than case hardening.
As an aside we used to use KE672 steel for making tools at work. KE672 and Gauge Plate were of similar ingredients but our toolroom 'much' preferred KE672 for its machining properties.
The pawl on mine is easily fileable so definitely not case hardened.
Phil.