The tractor is a Jacobsen painted blue with Ford decals. It's a good outfit.
A Kohler K241 at ten horsepower should have plenty of torque for a 42 inch deck. That engine should have a Carter/Kohler carburetor that has a #26 molded in the air filter side of the venture. When you throttle up from idle there should be a puff of black smoke. That tells you your setting is rich enough.
You could try a digital tachometer and make sure that engine is turning at 3600 rpm. That is where you have the full ten hp.
Make sure you can see daylight through the air filter. Those filters can look clean on the outside but still be plugged with dust.
Having the engine slow down when you engage the deck is a bad sign. Make sure all your pulleys are free, the deck spindles are free, and the belt or belts are lined up properly. Some decks are set up so the belt looks like it's lined up right but it drags on something when you are mowing. Don't ask how I know that.
Is the deck leveled properly? Are the blades sharp? Are the blades turning the right direction? Blade tips should go in the direction of the deck outlet/grass chute so they throw grass out the chute. Learned it all the hard way.