Chuck wagon motor swap belt help

[burndout]

Have a Chuck Wagon that blew its 2nd motor and the owner jist wanted it gone. I picked it up cheap and have 2 kohler 18hp v-twins and decided to put one in. The center to center clutch distance was 14 7/8". I setup the new motor with the same spacing but seems like the belt is too loose and the rear clutch isn't really spreading at full throttle. I know the belt is old and due for replacement, would it be better to get the same belt or to convert it to a comet belt? It runs the 40 series comet. I have some room to move the motor if needed, but dont see another belt that comes close to the 55.57 outside circumference. On the same note, if keeping that belt, what should the center to center be?