Having been unable to fix it myself, I've decided to sell my old engine. It's a Mariner 75hp 3 cylinder auto-lube 2 stroke serial number 9526970. It starts easily, runs, pumps water like a pressure washer and the trim/tilt works as it should. It also goes into and out of gear with no problems and the oil pump pumps oil. It has a few problems though hence its described as repair or spares. It has a tendency to overheat after 10-20 mins. I know it isn't lack of cooling, as the impeller, thermostat and temp sensor are all in working order. I had the covers off the water channels and they are all clear. There was some burnt oil on the middle cylinder by the exit port, and some scoring of the piston (had my local outboard mechanic look and he had no concerns once it was cleaned up). I believe it is running rich in the middle cylinder and needs properly tuned. I stripped and cleaned the carbs, replaced all of the gaskets including exhaust plate and end cover when it was apart and replaced the broken throttle links between carbs. It also needs a new rectifier. I really believe that this engine can be fixed with a good tune up, I'm just not the person to do it and my local repair shop can't fit it in for months (I have my eye on a new 90 Tohatsu anyway!).
The condition is shown below. It's a bit scruffy but reasonable for its age. The first image should be a link to a video of it starting and running.
There is a similar one on ebay at the moment with a starting price of £150 so I'm open to offers starting there, otherwise it goes on ebay.
It's now off the boat and sat on a pallet ready for collection Near Whitehaven, Cumbria. I'll include the fuel line, control box, harness, cables, bolts, security nut and keys. I'll even throw in the pallet. I have the facilities to lift the pallet into a lorry or trailer. Payment by cash on collection or paypal in advance if organising courier collection.