The old toilet was of the 'pump and dump' kind. Several mfgs, Jabsco, Rule, etc. You don't have a vac-u-flush, so you can use any type of older marine toilet you want. The only good news I have for you is Slickcraft built a pretty stout boat in the 70s.
However, I would guess like others that the transom will need to be rebuilt, and the outdrive is useless. There is a kit that can attach to some of the old OMC transom plates and mount an early Mercrusier Alpha outdrive. Suggest you check transom at the bottom first, then run water through the engine jacket second. If you are handy, the materials list will be fairly small, but the labor will be exceptionally high.
After all that repair is completed, you will have a $1000 boat sitting on a $1500 trailer. But, maybe it'll do what you want.