The engine project sounds like fun, actually. Model steam engines are cool too. The practical side of me is thinking "But but Brad, you could learn to mill receiver blocks and barrel vise inserts, grind square thread form tools, profile a barrel, ream a chamber, cut a shoulder, file (or mill) extractor grooves, crown on a lathe, file (or mill) sight and mag tube dovetails, bottom-tap blind 6-42 holes, and master the hot-water rust bluing process all at once with the Marlin project". Hell, if I can do it on a 7x14 hobby lathe with almost no prior experience you ought to be able to nail it.
You could also build a small propane blast furnace for cheap and sand-cast a puck of scrap cartridge brass to make your flywheel, just think of the money you'd save....