Engine overheat sensors

[Tarka1]

Any ideas on how to provide an overheat alarm on a Thorneycroft T90 (BMC 1.5)?
I have the standard VDO instruments which include gauges but no audible alarms.
Westerly Fulmar For Sale

 

[ms1]

http://www.indexmarine.co.uk/pages/extemp1.htm