As far as size i dont know the size of your cutout but 8hp sounds about right. Keep in mind that the cutouts in these bosts were designed around smaller 2 stroke outboards so you may have to compromise when shopping for a 4 stroke becausw of dimension and weight.
Perhaps worthless trivia, but perhaps interesting/revealing: In the early 70's there was, as you know a sailboat building boom because of the gas shortage and rationing of purchase opportunities.
Johnson/Evenrude responded by building the "Sailmaster" OB. It was their 9.9 hp motor, with a very low cowel, and usually sold in a long shaft version to sailors.
It was likely developed for the small fishing skiff industry, where anglers loved the much lower profile on their transom which was more out of the way for fishing. These were all lightweight two strokes. The first widely-available Honda OB's came along in about '75 or '76, and while people liked the quiet and thrift, they were heavy in comparison.
Sailboat makers were putting these low-cowel designs lon all kinds of makes and models, and especially loved the way it could tuck down into a well at the back of the cockpit, with a lid or cover over the top and/or front. Almost tidy; this also put the motor controls handy for manipulation, and best of all moved the prop under the back of the boat which kept it submerged even in small waves.
In comparison, the bracket method of mounting put the prop way out back where it caveated easily, and tilting those engines was sometimes quite a reach.
OTOH, being inside the transom area a bit could put that two-stroke exhaust and noise close to the occupants of the boat.
Among many others, there was the E-26-1, Ranger 24, Columbia 22, Cal 20, Catalina 27, PSC 20....
Then there were the bracket-mounts, as used on the O'Day 25. Yikes.
Re-boarding our 'way back' machine to nowadays, 4-stroke OB's are all we have for sale new, and they get lighter every year, altho not as light weight as we would wish, or as we recall from ye olden days.
The newer 4-stroke engines, have and have always had, one other advantage:
torque.
While HP is supposed to be calculated and true, I have found over the decades that my HP from my Honda and Yamaha outboards was a lot (!) more powerful than my older two-stroke outboards. I liken those horses to Clysdales compared to race horses....
It's probably a combo of more honest advertising by the makers and the lower-rpm torque curve. It's real, tho, in actual use.
FWIW, our new Johnson 6, in 1977, was referred to as the "buzzer" because it produced more noise than motion on our 20 foot trailer sailor!
Friends with heavier Honda 6 engines could zip right by, in comparison.
So, measure that well carefully and if possible find a local engine dealer who will let you borrow a couple of engines for a trial fit.
And finally, do not be depressed by these sorts of challenges.... short story: couple of months ago I was talking, at a distance, with one of our few club members who owns a 29 foot V-8 'express cruiser' rather than a sailboat, and he said he's planning to change over to.... a sailboat. He's tired of spend so much for fuel, and really has no place he wants to go to in a hurry. Shocking!
That Johnson model name was used for at least twenty years, and I found what resembles a 70's one, on the web.