The van is fairly immaterial. Look for the make of the roof light. There might be somthing written on the remains, or it might say in the handbook. There are only a few makers.
From there you might be able to find what you need from their catalogue.
At Caravan Crazy we specialise in the supply of genuine replacement windows, spares & accessories for UK and continental built caravans, motorhomes and campervans. We have over 35 years experience in the caravan industry.
Hi Doolander Welcome to the forum you could try, they might have some Burstner rooflights in stock but you will need the measurements good luck https://www.southdownsmotorcaravans.com