I think the Sportsman was OMC's entry into the popular tri-hull of the day. Engines were the 90 2 stroke like was used in their top of the line outboard and then the other engine I recall was the little Buick V6 putting out 155. Don't remember, but makes sense that the 120 was a little Chevy block L4. The boats used the OMC outdrive which boasted higher gear drive tilting, getting the LU out of the water for longtime marine parking over the Mercruiser hydraulic tilt of the day which OMC adopted later on when trim was added. Also the OMC outdrive had a large rubber boot between the outdrive and transom with the outdrive mounted to the engine and supported by the hull stringers whereas the Mercruiser was bolted to the transom and the drive and exhaust went through separate reinforced rubber bellows (tubes).
I know nothing about the boat's construction. However techniques in those days were wood that man or may not have been encased in fiberglass and the quality of the encapsulation was manufacturer dependent. Other thing is that over time with wave and trailer pounding, one has to question the ability of the encapsulation to remain intact....in short, you could have structural wood decay.
Have no idea as to parts availability. $100 isn't much but it could well be a sucker price. When doing restorations, costs just never end.....go to the restoration section herein and look at what's required to make a junker a star. I did a 1965 Starcraft 18' I/O with a 110 Mercruiser Chevy 4 and other than the deck and transom, which both had to be replaced, the rest was alum. which doesn't rot.....reason for selecting Starcraft. BTDT
And honestly, it's finicky but pretty stout. The clutch pack (canister lookin' thing just below the gear) should release the drive "up" to absorb impact and ease the hit. I met the bottom here in TN (mostly rock) one time with my Stringer, but I wasn't going fast enough to test the 35mph part out. But that's supposed to be how it works.
