Totally forgot to come back. I found the site and I have an answer. Is it the Scepter-Merc Outboard By Dan Odenberg?
You're absolutely right, and another point to you. I'm not sure how big the motor was - according to something I read it was a 6-cylinder 2-stroke, but most Mercury units used in motor racing were in the H-Mod category, which would insinuate under 750cc. I'm not an expert on these matters, but a sub-750 6-cylinder motor is in Japanese motorcycle territory. Maybe this was a larger unit.
PS just realised it ran in the F-Mod category.