Like pwnboat says: Because of the technical difficulties in replacing gears it is better and usually cheaper to buy a good used lower unit. The 150 can use MANY lower units but the best choice would be the dual exhaust lower units from a late Force 85 to 125 or, of course, a 150. These would be from about 1989 to 1994. After that, Mercury lower units were used and will not fit.
These Force lower units are all swappable. All use the same gear ratio and water pumps, and all have the same mounting bolt pattern, and in fact all use a two piece drive shaft. Single exhaust lower units can be used at a slight decrease in performance.
you CAN (if desperate) use a lower unit from a Chrysler engine from about 1979 on but these have a different gear ratio and will hamper performance. These are one piece drive shaft units and you must be certain that the drive shaft is the correct length to fit your engine.
