Hello, I live too far away to advise on a dealer, but you would be unlucky if you didn't find anyone local to you that wasn't familiar with those engines.
1) if it has an hour meter it is likely to be the silver coloured type supplied to either the MOD or RNLI by Barrus ( the main distributors of these)
2) The engine is part of what was a collaboration between Yamaha and Mercury to get the engines into the USA as the US would not buy Japanese for a long time. The name Mariner was born. Some engines are Mercury and some Yamaha, yours is Yamaha.
3) The twin cylinder engines were bombproof and run on a 1:50 mix. If you have a 25 litre tank this means you add 500ml of oil. Basically take the litre amount, multiply by 10 and then again by 2, answer in ML.
4) Any TCW-3 oil is fine, buy a decent brand though, motol, castrol.
5) If you are unsure of the engine, put it on a stand, fill a large container with water and run it (out of gear) in the water. Gearbox must be fully immersed.
6) As a rule of thumb, once you have primed the carbs with fuel, set the throttle and pulled the choke out, it should fire on under 6 pulls. If not push the choke in and try again. It should go, but make sure your kill cord is attached and not at your feet.
7) Engine should warm up and idle smoothly within a minute and pump lots of water. If not it may be a blockage or an impeller problem. No water means no cooling. Stop immediately.
8) If the engine runs well and is happy, I would change the plugs, put a new fuel pump and backing gasket on (you will never regret this expense). Change the gearbox oil (ep90) fill from bottom plug until oil comes out of the top one and replace top plug then bottom plug.
9) a bit complicated if you have no technical skills, but I would replace the impeller and thermostat.
10) There should be some red painted settings on the engine, this is the timing settings, leave them alone if the paint is unbroken. If it is broken, you will need a teccie who knows how to reset the timing.
11) Clean fuel filter. There should be an o ring and a seal. If either is missing it will allow particles to get into your carbs and cause a blockage. If the o ring is missing it can cause a leak and possibly a fire.
12) spray with GT85 bike chain water repellent all over the block.
13) if the engine runs well, the spark is ok, but it is an idea to check the plug caps or replace. Waterproof NGK ones on eBay are ok.
14) I you have a tiller, make sure cable wires are not corroded.
15) Drain down carbs, before starting (should have said earlier) any muck or particles come out, you will need a carb service.
16) Find a good inshore lake or river for you first run, it's better to test close to a bank than be blown out to sea, after an hour or two running you wi gain trust in the engine before going to sea. If you know of a harbour, run slowly around it for an hour. Moor up and stop and start the engine to see how easy it is when warm.