I know a guy that has made quite a successful software business out of abandonware. Find some business that's using software that has been abandoned. Original authors went out of business, etc. Maybe the users are still keeping MS-DOS or Windows NT computers around because it won't run on anything newer. This is not as uncommon as you might think.
Talk to the users, try to find out how many others are also using it. Rewrite it in modern technology, sell it back to all the current users (and ideally, also sell to others in the same line of business who might need it).
Talk to the users, try to find out how many others are also using it. Rewrite it in modern technology, sell it back to all the current users (and ideally, also sell to others in the same line of business who might need it).