20' seacraft is a great little boat. Smooth ride is perhaps its greatest attribute. '89 was a Tracker boat. gofastsandman might be able to offer insight on that era. typical concerns are the scupper tubes/ supports leaking, the gap between the cap and gunwales leaking, and the floors or transom getting soft due to water intrusion.
115 yamaha 2 stroke is good. yammie 4 stroke is a little heavy but workable.
When you employ a bracket you will usually end up raising the motor. rule of thumb is an inch per foot of setback.
If the bracket is not a flotation version I would probably shy away from the 150...
Check out bushwackers bracket treatise here:
http://www.classicseacraft.com/commu...ad.php?t=19779
Good luck! and welcome aboard.