Doing a conversion from standard to closed cooling could be easy if you use stock CC parts that are sure to be available on the used-parts market, but since just about everyone who had one experienced overheating, that's probably not the best way to go. Dave has some good links, and here are some other comments that may be of value to you. Good luck!
Paul
Related thread regarding oil coolers and heat exchangers:
http://www.network54.com/Forum/424840/message/1145302134/
http://www.network54.com/Forum/424840/message/1156960791