I purchased a flush mount ceiling light fixture which requires source wiring to have 90 degree Celsius rating. My home was built in the early 60's so I assume it has 60 degree Celsius rating. Can I install a junction box some distance from where the light will be installed, run the original source to it and run new wire (90 degree Celsius rating) from it to for the new light. The light fixture houses 4 25 Watt bulbs.

You may if you like, but not necessary to change older wiring to meet new code. If the existing exposed wiring is heat damaged, then may be a good idea to replace the wiring as you explained.