Standard 4.3 Lx mercruiser with alpha 1 outdrive I would think.
Picton's were built like the proverbial BSH ! , we had an outboard version, 16'6 with a mariner 200hp bolted on, still going strong today I believe (20+yrs old now), though it was only rated at max 150hp it proved to be an excellent combination.
I didnt think that that boat was too dear on ebay. If I'd seen it earlier (rather than within the last 25mins of auction!) Then I think it'd be on its way back here by now!