They are robust. Based on Yamaha 90 block, 3 cylinder 2-stroke. In service with RNLI usually set up as twins, etc.
Before spending anything on the engine, you need to determine its current health. Start it from cold and see how it idles. If the carbs are in sync it'll purr like a kitten after the usual throaty start (will need choke to start). Don't worry about smoke at start unless it's excessive, that could just be oil residue in the carbs.
Also get yourself a Draper compression tester to test cylinder pressure £20 or so. More details here: https://www.boatus.com/magazine/2016...ard-engine.asp
What you're checking here is the cylinder compression is equal across all 3 cylinders, give or take 5-10 psi or so. Anything greater, then you're looking at new piston rings or cylinder sleeves.
Assuming that turns out well, then factor in a full service. It's entirely possible to do it yourself but if your not mechanically minded, put it to a marine engineer.
1/ Drop gear oil to check for emulsification (sign of water entering gear box). Can be pressure tested to identify source of leak. Refill with fresh oil and new fibre washers.
2/ Replace impeller, visual check of water pump today see if scoring.
3/ Remove propeller, grease prop shaft and thrust washer, check for fishing line or other debris that can cause issues. Check propeller. Slight dings are fine, anything untoward then ship it off to Steel Developments to be balanced and repaired. New stainless steel split pin on fitting on castle nut.
4/ Drain carbs. On the engine undo the brass hex nut on each carb drain reservoir and inspect contents. Petrol and oil is fine. Any sediment, then carbs need stripping and cleaning.
5/ Remove thermostat and check water passage. Chances are there may be a sacrificial anode in there too. You can test thermostat in boiling water. New gasket required for refitting.
6/ Check sacrificial anodes (trim tab on anti cavitation plate, and under power trim unit). If serviceable, wire brush and refit, if like Swiss cheese, then replace. Don't use grease, dry contact required.
7/ New spark plugs, ensuring you don't just copy what's on the engine. Check with Yamaha for plug specification.
8/ Remove and clean fuel filter on engine.
9/ Check the electrical connections. Not unusual for bullet connectors to loosen or corrode. Especially 10mm nuts to starter motor live and earth points.
10/ Spray light coat of white grease on linkages to carbs ensuring safety R clips are in place.
11/ Grease gun with suitable marine grease on all grease points.
12/ Check battery condition with multimeter. If in doubt replace.
13/ Check fuel tank, hose and connectors (including 'o' rings and fuel primer bulb). If primer rubber pull feels hard to press then replace with genuine Yamaha or Quicksilver part.
14/ Check power trim fluid level and that it raises and lowers properly. Use ATR Dextron fluid, again check specification for Yamaha
15/ Check 2-stroke oil level, top up with Yamlube or Quicksilver
16/ Dismantle air box intake to inspect carbs and clean any oil residue. Also useful check of choke
There's still value in a Yamaha 75hp as its a mid-sized engine, and suspect that's where the value is, not in the boat or the trailer.