Repairs toro z4200 bogging engine

I am working on a toro z4200. Original kohler was replaced with a tecumseh 22hp v twin. Hydros are causing engine and blades to bog down. Blades are sharp, all belts are good, service kit installed on deck. Grass mowed is not tall at all. The hotter it gets the worse it bogs until it will barely move and it wants to kill the engine. Could the belt for the hydros be too tight somehow? Motor runs like a top so I pretty much think its in the hydros somewhere just dont know where to go with it. Pull them, drain and fill with new oil? This one has me stumped. Any and all help would be greatly appreciated. Toro m#74360. Hydro motors left m# zc-dfbb-5w5a-3cpx numbers on right motor are faded but its original to the mower. Engine tvt691 600816a