OK, I found a replacement heater core that fits in the plastic heater box: https://www.carparts.com/Heater-Core/Replacement/REPM503005
I'm looking into using Sharkbite 1/2" 90 degree elbows to reuse portions of the old piping: https://www.amazon.com/SharkBite-Connect-Plumbing-Fitting-UR248A/dp/B0C4QWMXST/ref=asc_df_B0C4QWMXST