A few people have put a Cummins in their H1. Do you want a 12 or 24 valve engine? The 12 is alot easier to put in. As far as fit goes, you will need a 4-5 inch body lift, and a custom engine cover. Did you check out this website yet?
http://www.avalancheengineering.com/hummer.htm
They offer the conversion, perhaps they also offer a kit?