Parts are available, but you will have to look.
Iboats has them:
iBoats has outboard motor parts and engine parts for brands like Yamaha, Mercury, Johnson, Evinrude, Volvo Penta, and more. Great prices and fast shipping.
ww2.iboats.com
And so does Wholesale Marine:
Shop for essential OMC parts and OMC sterndrive parts. Wholesale Marine carries OMC anodes, cooling systems, fueling systems, and more. Order today!
www.wholesalemarine.com
I believe you may have a rare sort of animal, as that the best of my research has shown that Fibreking only made one sterndrive bomber of 18 feet (1988), and another called the Vanguard at 19 feet.
I currently run an 89 Marathon that went out of business years ago. Don't be discouraged. older boats have a certain "charm" about them. Mine runs perfect and is a ton of fun. Mercruiser Gen 1 stern drive.
I'm guessing, again, that your motor is probably a small block chevy.
I'm guessing your boat looks like this one?