A word of caution to anyone planning an LPG conversion...
Renault officially provided the LPG option in the late 70s / early 80s on the 1108 engine, but among the necessary conversion parts, they also listed a different cylinder head and exhaust valves.
This must had been because LPG promotes valve seat recession and/or exhaust valve burning more rapidly than unleaded petrol. With LPG we can burn much leaner mixtures as the mixture is by definition more homogenous (no droplets to atomize and spread uniformly in the combustion chamber here...). In fact, all LPG systems are being set lean, to offset a bit the higher lt/hr consumption of LPG versus petrol (expected, as LPG has a lower thermal power than petrol).
Add to this the fact that there is no liquid vaporization inside the combustion chamber (a useful side effect of petrol, that lowers combustion temperatures) and you may see that exhaust valves and seats have a hard life...
This may not be the cause with Andrea's engine (low compression), but it may be a problem on a GTL.
Note that all LPG conversion kit manufacturers list various valve problems (exhaust, but also inlet in some cases!) that appeared in modern engines, when using LPG without valve / seat modifications, some of them after the first 30.000 or so kilometers.