what temp is too hot for a brushed modified motor?

Here is the old school answer. After a run if you can keep your finger on the can for 5 seconds it is geared correctly.

watching the color of the windings and the com. is a better way to see if things are right but you need to know what your looking for. Honestly only a seasoned racer can tell this way. I know this didn't help you any.

Both of the above are kind of correct.
When a brushed motor heats up too much, the magets weaken (also for brushless), but also the comm gets too hot. When the comm overheats, it will change colour from lighter reds and go through green and to purple. When that happens, you can pretty much throw it away.
Also, the brushes will glaze-up real fast as the motor gets hot, which reduces their performance considerably.
If the can is going over 55º, your starting to push it in most cases