That 140 was de-rated to a 115HP rating. The changeover from distributor ignition to ADI ignition occurred in 1979, so that 140HP motor could have either one. You want the ADi ignition, as it is used on much newer motors, and parts are cheaper.
If it were me, I would look for a late 90's V6 motor to replace that IL6. You should be able to get one for $1K or so. The V6 motors are much better, as they have better power from idle all the way up to max RPM.
My Merc 1500 IL6 had little power until RPMs hit 3000. Of course, then she would accelerate from 30 MPH to 50 MPH in a few seconds.
That was the nature of the beast, wild port timing and big carbs made for that type of peaky power band. My experience with the inline 150, was it would idle very nice, but from 2000 - 4000 it was a cantankerous thing, once it hit 4000, things happened very fast and it had a lot of power.
A V6 150 is far superior, when it involves low end torque, and burns less gas despite a 22% increase in displacement
As for the 140 becoming the 115. Maybe, maybe not. The 140 had a WOT range of 5300 - 5800, the 115 was always 5000 - 5500, prop or crank rated. Carb calibration was different between all three engines.... In 1984 Mercury started using a piston with a lower dome, How that affected the HP, ???? My 84 115 gave me a 11mph increase in speed over my 78 90, so that difference suggests a 50hp jump from the crank rated 90.
