that's really not a suitable motor for speed control. It can be done, but it's not going to be light or cheap. Far better to get a retrofit kit that is made for purpose. I have worked in motor control and have a collection of small brushless motors and I have considered trying to use one of them. But it's just not worth it with the systems I can buy that already have all the engineering done.
I also don't think the power source is the important part of the legal power limit. So just because it's coming from a generator doesn't mean you can cheat on the limit. If I were really worried about it, I would just take the name plate off of it. You probably aren't going to get 1hp out of that motor anyway.