I have no idea about the Fulvia but have in the past made a tool for my Twin OHC Salmson which replaces the spark plug and has an L shaped hook which can used to hold the valve up while the spring is removed and the seal replaced. The shank of the hook is mounted eccentrically in a bush which screws into the spark plug hole so that when the hook is turned the hooked part swings outboard of the bush inside the combustion chamber just enough to catch under the valve. A coil spring around the shank of the hook holds it up while you fiddle with the valve and seal,
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Ican post some pictures if that would help.
Mike Clark