From what I have read, the way horsepower is “calculated” has changed from the days of the K series to now…. So, no an 18 horse now, is not the same as an 18 horse K series from back in the day…. (Yes Kohler made a single 18hp K series, IH just didn’t use them…. I think Bolens did)…….
Also, if you look at some new engines, they are sold by CC’s of displacement, and not a horsepower rating…..
An example…. I have ran, and IH sold them this way, a 44” (44A) and a 50” (50A) deck on a 149, 1450 and 1650. All 3 handled them well, and the tractor did not seem under powered for them, and they did a good job….
Now go look at a new Cub Cadet… even the smaller deck tractors (46ish inch) deck all have 20+ horse twin cylinder engines. My 54” Zero Turn has a 24 horse Kohler Twin…. But you look at a super that could run a 60 inch Haban deck…. They were what 20 horse max…. They were twins… but “less” horsepower…..