The problem with using a dirt bike fender is they are made for 21" wheels, and they are mounted up high....they would look pretty silly mounted down on the tire as the radius just wouldn't be right.  They are also made to keep the dirt from being thrown in the riders face...they don't do much to keep crap off the engine.

Here is an option or two for a longer fender.  The KZ750 one would mount with just drilling new holes - the Honda Goldwing fender is bigger - but would require you making a new mounting bracket.
http://suzukisavage.com/cgi-bin/YaBB.pl?num=1453983731/0#0

Sorry.....somehow I missed the dirt bike fender was going to be cut up for a mud flap, and not used as a fender.

This is a little more involved but I used a fender off a sportster,mounted to my zuk fender (mig welder)  then bowed out the bottom near the ground (acts like a mud flap ),I rode 100 mi. today it some wet roads(scattered showers) that use to soak me to the knee, and didn't even get my boots wet.