A thermostat stuck open will hamper performance as the engine will be running rich due to it not being up to operating temp.
The engine serial number is located on the front of the block to the right of the oil filter although with the age of these blocks now it is probably unreadable. However, the letters 'J4R' are stamped into the rear of the block, and the thermostat side of the head should have 'RS' cast into it. That's how you know it's an XU10J4RS.
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"Supercharged - 454.1bhp/317.5lb/ft"Peugeot 306 GTi-6
2000 (X), Moonstone