Bujutsu is the collective term for the practical fighting systems of Japan. The principle aim of these arts is to develop practical fighting skills in their practitioners. Unlike the Budo, these systems are not concerned primarily with any form of spiritual development but are concerned with pragmatic effectiveness.
The only real difference between the classical and modern alternatives are that the weapons and circumstances studied have adapted to fit with modern circumstances.
For more information related both koryu and modern bujutsu please refer to the following (Click on book images to purchase these from Amazon.com):
Donn F. Draeger's Three Volume Set - The Martial Arts and Ways of Japan
Volume 3 of the set includes a section on Aikido.
Koryu Books - The Classical Warrior Traditions of Japan Series, Edited by Diane Skoss
For additional information regarding Koryu, please visit Koryu.com