What kind of motor would help. Look around lower motor mount for corrosion. Do a search or look lower in this forum. There was a discussion the last two weeks. If the lower motor mounts are corroded they put the tube the shift cable rides in in a bind.
Im thinking that where the shift rod goes into the lower unit may be corroded. I think there is an o-ring where the shift rod goes into the lower unit. I have seen salt and junk get built up there causing the shift rod to get in a bind and not move freely. If this is the problem you would have to disconnect the shift rod up top between the power head and the motor pan. then drop the lower unit. Then I would take the shift rod, pay attention to what direction it is facing then screw it all the way down counting the number of times around. This will tell you where to put the shift rod back at before reinstalling. Once you do that pull the shift rod all the way out and then pull the housing that the shift rod screws into. Clean all the junk out of there, probably replace the o-ring or o-rings i think there may be 2 then put it all back together. Atleast that is what i would do if that were the problem.
I guess it would depend on if you had it done or if you did it your self. As far as parts your not talking much at all. Probably just the cost of those o-rings and maybe a gasket. Probably 1.5 to 2 hrs labor (just a guess). If you have it done you may want to consider a water pump job while you are at it. Just easy to do if you already have the lower unit off.
Amybe you have the throtle pulling back to tight, on the stop against the engine cause the control to bind, before going into shift, check the shifting my hand spin the prop and move the shift lever by hand should move easy and go into gear, but spin the prop while shifting,, other wise need to know more info on brand of motor!