For the most part there are three major components involved here - a motor, a transom assembly, and an outdrive. If you plan on keeping the Mercruiser drive on the forrester, stern cut outs aren't a concern, because you aren't changing anything on the transom assembly. Now it's likely that the bell housing which bolts to the engine will have to be used from your existing Merc motor - but I would be very surprised if it didn't bolt right up to the donor motor. And possibly the flywheel. What are the years of the two motors involved? According to a web site I just looked it, in 1987 on this motor GM changed the rear main seal - so you can't take a flywheel from a pre 87, and put it on a post 87. But i would be very surprised if you can't use the flywheel on the OMC motor with the merc transom.
There is also a coupler, which connects the shaft from the out drive to the flywheel, again,you will be using the coupler from the Merc -or better yet, buying a new one.
Once you get the big steps above done, there are some more minor components, that you may have to swap from your Merc motor onto your OMC donor motor - here it depends on which are in better shape. If it's like the 5.7's I dealt with, the starter and solenoids are set up a bit different - so you'll have to choose which you want to use. You'll need the shifter plate from the merc boat,as those are a bit different.
You only need to be concerned about transom cut outs if you plan on taking the OMC drive with the motor to the recipient boat; and you would only need to get an adapter if you wanted to bolt a Merc drive to a transom that previously housed an OMC - but it doesn't seem like that is your intent.
good luck - I dove into a similar project with very little experience or skill, it took me a while, but turned out fine.
