1985 mercury 35 hp

The motor runs great although I feel the throttle isn’t opening all the way. The main problem is getting it back to neutral when coming from drive. Sometimes it works and other times I have to shut off the engine shift it then restart it. Is this a cable problem or something internal.? I have changed the throttle and the cables. Disconnected from the engine they work fine. Just one more question. I can buy a 1986 mercury 50hp cheap. The motor is junk but it has the hydraulic lift . Will it work on my motor ? How much of a job would it be to change? Thanks