Am new to the site and the camping scene (so be patient if questions seem naive

Anyway am based in ireland and posted this on another form there but was recommended to come to you guys.
Basically we bought a 1990 reg westie van last week and were delighted with the purchase. We were told initially that there was no LPG in the van as when we the commissioned seller (doing it on behalf of a friend) tried to test the cooker etc it wasnt working. He checked the gauge underneath which appeared empty and he informed us of that. When we got back we went to the local garage the the attendant could only get a couple of litres in and he told us it wouldnt take anymore yet we were under impression it was empty.
Anyway today we were getting ready to take it out on the road today when we got a strong smell of gas. We opened the plastic cover underneath and the pressure valve had blown off its red cap. Gas engineer told us the van was probably very overfilled.
This has caused us problems as it appears the lpg gauge is broken as it read as empty when it was already full. Worryingly also is the fact and excuse my ignorance on this but are westies meant to have a mechanism that stops you filling beyond 80% of tank capacity?
To make a long story short im wondering are replacement gauges hard to source and get fitted? (in ireland!) Is it possible to fix the tank so it doesnt let us fill beyond the 80% of capacity and someone also informed me that with some campers once the pressure release valve is activated that too might need to be replaced but he was fairly sure that was for older models and not the 1990 model? Many thanks for any advice!