The B&O Railroad museum has possible the largest collection of original or replica pre-Civil War American steam locomotives, with a smattering of other "Era 1" engines scattered at other museums around the country. The earliest date, btw, is more 1830s than 1820s in the US. The first locomotives here tended to be 0-4-0 with a 4-wheel "tender" for fuel and water. A 2-4-0 was the improved design for better tracking. I'm fairly certain that we had a 2-2-2, but I want to say that we're talking about single-digits for examples, and probably only one or two railroads. There were also 4-2-0 and 6-2-0 designs.
A 2-4-0 or 2-4-2 might be your best option for scratchbuilding, as you could use an HO 0-4-0 as a starting point. The search term "antebellum steam locomotives" might be helpful. There are a half-dozen or so books on early American steam locomotives, or early American railroads, that show up on eBay now and then. There are also several railroad museums with extensive archives where you might find photos, diagrams, or even preserved engines.
You are largely working outside of the normal range of modeling eras, but the upside to that is this was an era of unique designs, so let your imagination go wild. Also, don't be afraid to think outside the box. That era had 5' and 6' gauge railroads, so you could use a G-scale mechanism & track with an O-scale body for a wide-gauge railroad.
I hope some of this is of use to you.
A 2-4-0 or 2-4-2 might be your best option for scratchbuilding, as you could use an HO 0-4-0 as a starting point. The search term "antebellum steam locomotives" might be helpful. There are a half-dozen or so books on early American steam locomotives, or early American railroads, that show up on eBay now and then. There are also several railroad museums with extensive archives where you might find photos, diagrams, or even preserved engines.
You are largely working outside of the normal range of modeling eras, but the upside to that is this was an era of unique designs, so let your imagination go wild. Also, don't be afraid to think outside the box. That era had 5' and 6' gauge railroads, so you could use a G-scale mechanism & track with an O-scale body for a wide-gauge railroad.
I hope some of this is of use to you.