No milage limit for classic or antique tagged cars - in addtion, the code states: "...maintained in or restored to a condition which is substantially in conformance with manufacturer specifications." That means it
can be modified, but not such that it's not recognizable as the original design. So you can swap engines, built it up to 1000hp, change tail lights, remove molding, etc. But if you are running a tube-chassis car w/ a lexan (or whatever it is) funny car one-peice body, I'm pretty sure they would consider that not in conformance w/ the manufacturer.
Here's the actual PA Vehicle Code:
http://www.pacode.com/secure/data/067/chapter67/chap67toc.html