thermostat

I've just driven down the A1 and been stuck in traffic jams several times. The next day I lifted the bonnet on my 2009 GV Ddis to find that at the top of the radiator a hose link had leaked and sprayed the engine compartment. My first thought was "I probably need to change the thermostat" but...

I am fairly new to the hobby, but bought a 95 Geo Tracker a few months ago. When I bought it, the previous owner said the owner previous to him had swapped the engine "for the bigger one". I said great, assumed it was the next engine up (didn't research), and it ran and drove decent for a great...