55 sounds way too low for an '83. That year, they were pulling 6 horsepower more than the year before, which would put the '81/82 at a measly 49. That's quite a bit less than the old 501, and not too far from the 42hp claimed by the '81 250. I think I recall Super Hunky saying it had 63, which sounds about right given that the '81 is typically quoted in the 54-57 range, if I remember correctly. I've found, though, that horsepower readings tend to be really weird. Like, magazines saying that Husky's 390 makes less than 40 horsepower, which is less than a late-model Maico 250. I've ridden both, and I can tell you right now that the 250 ain't got no 390-beating power.
Makes you wonder how that 67hp number is on the new 500, if you were to put it on 10 different dynos and test it. I've always wondered about that sort of thing.