Building one would be one way to go, but since TV went digital, thousands of high quality modulators got dumped onto the surplus market, especially when local CATV systems upgraded.
I use a Blonder-Tongue AM60-860 unit to provide OTA material for my vintage TVs. It won't cover Channel 1, but it does cover broadcast channels 2-83, as well as cable frequency assignments.
Do a search for "agile modulator" on eBay, and you can find one for little more than it would cost to build your own, and it will be a broadcast-quality piece of gear made to run 24-7 for years.