I wouldn't worry too much about the derailer hanger.
On a steel frame bike it is possible to bend the derailer hanger back into alignment, as long as the damage is not extreme (i.e. the hanger is not broken off). Steel can handle this sort of abuse.
However tolerances are close, so it is advisable to have your LBS do the repair. They will have a special alignment gauge that allows achieve the proper adjustment.
Replaceable derailer hangers are the rule on aluminum frame bikes because aluminum is not as resilient as steel and cannot be bent back to shape without breaking.
I'm not familiar with the Jamis bikes in particular, but I've never seen a steel frame with a replaceable hanger before, either modern or vintage.

Good luck in your searches!