Umihara Kawase Level Editing
Over the years, there have been a number of attempts to analyse the inner workings of Umihara Kawase, with the goal of creating a level editor. The result of this is Riverback, a level editor developed by Commando125. Riverback allows for the editing of a level itself as well as its settings, including but not limited to enemy types, level timer and which levels exits lead to.
For more information and discussion, see the ROM hacking thread on the forum.
Note that, if you choose to become involved in the creation or play of fan-created levels, KawaseFan.net does not endorse piracy, and encourages you to use a ROM backed up from your own Umihara Kawase cartridge.