Re: 1985 125 force vs 1974 115 evinrude
Performance - wise, it will probably be a wash. the Rude will probably give equal top speed and acceleration--assuming it is running well.
But my question is: Why would you want to put on an engine so much older? Has the Force crapped out and you have the Rude lying around? Understand that the Force is relatively easy and inexpensive to restore to running condition if you do the work yourself.
Realize that with the Rude being so much older it will have more corrosion issues (assuming it has been regularly used and not lying in a garage for 15 years) and is a heavier engine. You will need to change controls, some electrical, and probably the tach, and the Evinrude is a bit more of a complex engine to work on should something happen. And, It is still a good idea to change the water pump impeller and lower unit oil. Also if the Rude has the electric shift lower unit, they have a bad reputation and parts are not available for repair unless you can find a junker.