Based on the qc sticker on the exhaust cover it looks to be a 1987 model (possibly sold as a new 1988) - and it's general design, decals and cowl would support that.
This particular models "big sister" the 90 (which used the same block) was/is probably one of the most successful mid-range horsepower motors of all time. It didn't get that distinction from being just a so-so motor.
Quite a number of parts for the early model 3 cylinders, such as this, have been discontinued, but the important stuff is still available. Carb kits, waterpump kits, ignition components etc are still very much available.
I agree with you, this doesn't look like it's seen salt.
Trying to determine value can be tricky. To a dealer in North America, as a potential trade, this is worth practically zero - nobody wants an "old" 2 stroke. While a dealer may take it trade and even give you something for it, you can be almost guranteed they would add the trade-in price into the total bill for the new motor.
If we look to the NADA guide, a 1987 75 ELPTO (electric long shaft with power trim and oil injection) a value of between $140-160. It originally sold for $4800 back in 1987. Think of this as kinda the bluebook value - what insurance would cough up if it was submitted in an insurance claim.
It's true value is somewhere between what the seller wants and what you are willing to pay. Plus, you have to factor in the cost to get it "usable".
Cosmetically, obviously it has had a mishap - so knock off at least 10% for that. Unless the current owner can show that regular maint has been done (backed up by receipts - I wouldn't even trust the word of my father when it comes to a used motor) you have to count on 1) cleaning/rebuilding the carbs 2) waterpump service 3) sparkplugs and gear oil.
If you can do the work yourself count on spending 1) 100 bucks 2) 100 bucks 3) 25 bucks.
If you will require a shop to do it, then including parts it would be 1) $500 2) $200 3) $150.
So figure on between $225 and about $850 to get her "seaworthy", assuming nothing is really wrong with it.
If I have the year correct the motor is damn near 25 years old. So is it worth 1/2 it's original price if it's "perfect"?
Using that as a baseline, let's say 1/2 price would be $2400. Deduct 10% for cosmetics = $2160 then subtract initial service ($850). I come up with $1300, and for me that would be the extreme high end. For that price, the compression better be excellent, the ignition working properly and I would have to at least see the motor running (even if it needs a tune-up).
If it won't start but it otherwise looks ok and the compression is good, the value would drop somewhat below 500 bucks.
My reasoning may be a bit confusing, but that's how I come up with my numbers when I'm trying to value an engine.
Hope it helps.
