4 holes in the transom? Top holes are spread wider then the bottom holes? Holes are 1/2 "?
That boat was designed to carry a motor large enough to not have transom screws, But you may have just two holes, because of a motor that did have transom screws. The two holes were for security reasons. 1, takes longer for someone to steal it, and 2, it insures, if the transom screws work loose as they sometimes do, the motor won't jump off the transom, as they will do.
Bolts long enough to do the job, washers on the head of the bolt inserted from the outside. Large fender washers on the inside under locking nuts or nuts, lock washers, and fender washers. Plenty of caulk in the holes and fender washers.
When you buy this motor, you need to get the right length leg on it. Measure the transom from top to bottom. If it's real close to 20" then that's the length motor you need, long shaft, if it's real close to 15" then that's the length motor you need, short shaft.
If the previous owner had a long shaft engine, 20", and that boat's transom is 15", that would be the scenario for a jack plate.