Well, since you have it, the best way to find out is to clamp it on, and go test it. However, I am saying close to 5 mph at the most. I am estimating my 4 horse kicker on my 1542 is pushing me right around 3 mph, judging by the test run.
I bought it for only $60. He said in ran fine but it was in his garage for two years. I figured if I can't get it running, I can always just sell it for either parts or scrap.