I don't have any knowledge on the Fleetline Sapphire but how did it perform with the DT60?  The weight of that motor is 104kg according to Google so I would think the 60HP t0 75hp range would be fine.  With a boat that age it really would pay to get the transom checked out.

The reason I’m wanting to swap the Suzuki dt60 is because they are so rare and replacement parts are nil. It goes good and the transom seems like it’s in good shape. I’ve also got a leak in the lower gearbox that’s letting water seep into the gear oil which isn’t fixable. The seals are fine but the plate itself has so much corrosion that it doesn’t seal so out with that motor and after something that is in high supply.

The only reason I was asking if I could push the limits is because Yamaha 90s come up all the time for reasonable pricing. I’ve seen a few on the same model of boat so might give that a go and reinforce the transom with some thick aluminium plating.

4 stroke technology is definitely not in the budget for this oldie as well as losing power because of them being so much heavier per hp

The boat itself is pretty damn heavy and I think any smaller than a 60 would be troublesome.