As mentioned your engine is carbureted so other than checking engine hours a scan tool won't help much as there's no connector to plug in for other engine data parameters to read.
The over-temp warning horn ought to come on right around the 196 degree mark which is the sender with the Yellow and Black wires....The manual shows the yellow and black wires as an oil sensor/sender which is a misprint within that particular TKS service manual.
You mentioned you replaced the thermostat, did the warning horn sound before or after the new thermostat?
Did you use a OEM gasket with brass type rivets that run through the gasket? And if so did you install the gasket on dry (no sealant)? Due to if either situation occurred during installation then they could affect the warning alarm system due to loss of good grounding.
If the above info doesn't seem to be a known concern for your situation and the alarm hasn't came back on for some time/at all yet while the engine is running especially around when you first experienced the horn, 'Then there's a decent chance the horn may not come back on for sometime to come since you already unplugged the temp sensor wiring as there have been other similar incidents / reported experiences of a ghost type alarm with these TB5 (Thunder Bolt 5) ignition system engines within around your engine model years.
The fix for them was/seemed to be unplug all the connectors to the ICM (Ignition Control Module) and temp sensor because as you know the yellow wire runs to the ICM which is part of the overheat alarm system too.., 'Check/inspect/clean the wiring connectors ensuring the wire pins are making contact by carefully/softly pushing wires inwards while reinstalling the plug connectors and usually you won't experience what seems like an intermittent ghost overheat alarm for sometime / if ever. If the warning horn comes back after a long while, unplug it again and see if it stays off for awhile.
