Carolyn: a new 6-8 hp engine would answer your needs. You will have to buy a four stroke since they don't make small two stroke engines anymore. You can get by with a little less horsepower with a four stroke because they generate more torque than their equivalent two stroke. A couple of caveats: get the longest shaft engine you can find. Most manufacturers consider a 20" shaft to be their "long shaft" model. A couple of them offer a 25" extra long shaft. For my money, the extra long shaft is the way to go.
Also, be careful with the mounting bracket. Most brackets are not rated to handle four stroke engines. If your 1979 boat still has its original bracket, you can be sure it is not rated to handle a four stroke. There are two reasons why four strokes need the stronger brackets: four strokes tend to weigh more, and generate more torque than their two stroke hp equivalents. Once you commit to changing the bracket, you bring up a number of potential issues, such as: the footprint of the two stroke bracket is almost certainly smaller than for a four stroke bracket. This may mean you will need to change/augment the existing mounting pad (that angled block of fiberglass between the transom and the engine bracket), and potentially drill new mounting holes in the transom. Nothing unsurmountable, but a pain nonetheless.
For my boat (an Oday 23), I replaced an early '80's model Johnson 9.9 with electric start in 2002 with a two stroke Nissan 8 hp electric start model. I went with a two stroke over the four stroke for a couple of reasons: a four stroke would have required me to replace the engine bracket and mounting pad. I had just replaced the mounting bracket the year before and didn't want to have that investment go to waste, and I had rebuilt the old mounting pad at the same time. Perhaps more importantly, the Admiral weighed in on the side of a two stroke; she thought the four stroke would be too heavy for her to tilt up or down.
Since then, the industry has stopped making small two stroke engines, and the weight of four strokes has come down considerably. Can you get away with using a four stroke on a bracket rated only to handle a two stroke? You will find that question debated multiple times on this site. I don't think there is a clear answer, although my hunch is that a new bracket rated to handle a 20 hp two stroke should be sufficiently robust to handle an 8 hp four stroke. But I'm not an engineer.