... should've looked a little harder before making another post literally 10 minutes later, but CR D27 (or 230th street) east of Ira Ave to IA471, follow south to CR D36 all the way to US169, US20BusFor between US 169 and CR D20, CR D20 to CR D25, CR D25 to CR R75, CR R75 to CR D20, CR D20 to US 65, and CR D15 to IA 57 are all designated as US 20 Historic.
There's signs up periodically throughout the route, I didn't realize how long it was but the route looks something like this:
(Link)