There are no balance gears in a 12 HP AQS Kohler which is what should be in your 1250.
Try turning the ISO mounts upside down so the good ones are on the bottom.
There IS an upper and a lower in an NOS set w/2 thick washers that sandwich between them when assembled.
When the bottom mounts go to pot and the engine settles onto the X member this throws the driveline out of alignment and causes the rag joints to run crooked, hence the vibration
While you are in there, make sure your pan to cradle bolts are tight and this would be an excellent time to do the cradle mod and add the cross brace
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