If it loses power gradually, then the problem may be elsewhere, such as overheating or a fuel delivery issue. If it quit suddenly or quickly dropped to about 1/2 speed, then it may be an ignition problem with one cylinder cutting you.
If you are sure that there is no spark, then each item of the ignition system has to be tested and/or inspected per the steps in the factory service manual.
What is the model number of the motor?
You can purchase a service, parts, or owners manual using the model number and ordering from:
"There is never just one thing wrong with a boat";
-- Travis McGee, main character in a book series by John D. McDonald
The factory recommends that a properly trained technician service your Johnson or Evinrude outboard motor. Should you elect to perform repairs yourself, use caution, common sense, and observe safety procedures in the vicinity of flammable liquids, around moving parts, near high-temperature components, and working with electrical or ignition systems.
The information offered here is only general in nature and should not be construed as complete factory approved procedures, techniques, or specifications. Always use the proper service manual for your motor, up-to-date service literature, the correct tools, and have an understanding of how to proceed with troubleshooting and repair methods. If you are unsure or uncomfortable with a procedure, a situation, or a technique, enlist the services of a factory trained technician.