It might be good to also revisit "assumptions" about your present drive train.
A lot (!) of stock sailboats have incorrect props installed, mostly from prior owners, but some from the factory.
If....your boat cannot hold it's calculated -displacement- speed against a moderate chop or headwind at the engine's rated cruising rpm, I would first take a hard look at the prop that is on it now.
A ton of sailboats have a fixed two blade that is not well optimized for the boat.
With that waterline and stock engine, I would expect 6 to 6.25 kts in flat water with no strain on the engine. Assuming a smooth bottom, of course.
Heck, we used to motor at 6 kts all day long with our 10 hp Honda on our prior 4K displacement 26 footer. That was after I had the prop re-pitched, BTW.
Anyhow, more HP is probably a great idea, but just be certain that the engine is not the only impediment to speedy progress.