Don't you have any backup of it? Even the email it originally came with? You should always backup your keyfiles and other registered stuff to several places.
But anyway, contacting to MorphOS team should solve the issue...
A practicable solution is to inject the MorphOS key via RegisterISO into the MorphOS ISO and burn it on a CD. This will provide you with a very good emergency Live CD for rescue or backup purposes, and it's registered.