I ran that same 75 hpengine from 1976 for years. Mine was a short shaft with a small gearcase, however. Your version has the standard 20" midsection with conventional gearcase. It's all electronic ignition, which is solid. I prefer the later 3 cyls which have adjustable idle screws-yours has fixed jets. Seems these 3 cyls are just not good idlers at low rpm's. A solid engine, good runner. Your version suffers from two problems. 1. It's only 49 cubic inches, yet is rated at 75 hp. That's pretty small displacement for the hp it's rated for. It is rated to run up to 5750 max normal rpm's-higher than the sister 70 hp. 2. The powerhead is ported for high rpm hp, so only makes much hp in the upper rpm's bands. So, taking both items into account, I agree with Boobie's comment that it will be a dog out of the hole. A strong runner over 4000 rpm's. They made these by the thousands and many parts are still available for these engines. If you have no controls, be careful. OMC changed the throttle and shift cables in the 79 model year. That means the control boxes for 79 and up are different-internally. Look on ebay for a used control box. You did not mention whether the engine has power trim. Maybe not, but it's a really great addition if it has it-and adds extra value to the engine. (That will also make the control box different-if it has power trim you will need the switch in the control box handle.) If it does have trim, you are probably missing the special trim control box which you will have to get used-it's NLA.
