Not sure if you tried or not, but a good cleaning and carb kit may do the trick. A lot of your local mower shops may have the carb kit.
I have bought a few HM80 and HM100 carbs off ebay. The trick is to look at the throttle lever and choke on top and fuel inlet on the side of the carb to match yours as well as the adjustable jet on the bottom of the carb is nice to make fine tuning easier for top end power.
I have bought a few HM80 and HM100 carbs off ebay. The trick is to look at the throttle lever and choke on top and fuel inlet on the side of the carb to match yours as well as the adjustable jet on the bottom of the carb is nice to make fine tuning easier for top end power.