Yes, an M2 gearcase (1.85) will bolt on but you will have to purchase a different shift rod. If it is not a new gearcase, then you may have to exchange the driveshaft with your old one depending what shaft is in it.
There is also a kit that is needed because of the much higher water pressures that the M2 develops plus you may have to replace the pressure relief valve on the motor.
It is best to work through a dealer as there are several service bulletins about the procedure and the additional parts that may be needed.
"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.