I would look for a dealer and find a scag that has your new motor in it and order a new scag harness for that machine if not troubles will follow. I’ll will try to look tomorrow for a part # in a machine that has a 850d in it.
You have to call Midwest tell them your machines numbers and tell them what you have done and see if they have one or for your engine and scag mower. They prob can tell ya quickly if there is one or an adaptor for that repower.