Any Merc/Mariner dealer should be able to get you the key. If you have the key number then great, in not then take the control box or ignition switch with you and they will be able to match it up with their master set and get you one.
ps. I have the pdf workshop manuals for all the newer mercury/mariners including the 4 cylinder 30/40hp, or 3 cylinder 40/50/60 but not the older 4 cylinder 50hp. But if you need any info I can scan it from the manual at work