Bill Higham have a few Yamaha 75hp that they can fit carbs from Yamaha 90 and new decals. https://www.billhigham.com/boat-engi...-left-call-now
I've got a soft spot for these engines, good grunt from a standing start, easy to maintain, lightweight, smell of 2-stroke oil in the morning.
What was wrong with the 70hp? Surprised you never got to the bottom of it. With these things, it's the difference between a mechanic who's been around 2-stroke engines for years and can simply hear them at tick over. Was the timing and throttle all reset when the issues started?