First off if he did in fact tell you that it would screw up your
boat there is the clue of FOS. It might screw up the motor but not the boat, which I seriously doubt. I have converted engines from pull start to electric start (using homemade fabricated parts), and tiller to stick stearing and never had a problem. NO engine that size NEEDS (but nice to have) a Tach, that I know of. Now some of the larger newer engines might need the tach as it might be part of the wireing system, reasons being...I'm first not sure of and too lengthly to type.